DOC: confusing docs about combined log file

What's going wrong?

I discovered this default behavior of PM2 by trial and error:

1. -out logs and -err logs will default to output in ~/.pm2/logs, unless and only unless they are rerouted or disabled via configuration or CLI.
2. In addition to -out and -err logs you can create a combined log. Creating a combined log does not override or disable -out logs or -err logs. But unless it is configured a combined log is not produced.

How could we reproduce this issue?

Supporting information

Simple fix idea:

modules.exports = [{
  script: 'echo.js',
  error_file: 'err.log',      // PM2 will always output a -err file, unless you disable
  out_file: 'out.log',        // PM2 will always output a -out log unless you disable
  log_file: 'combined.log',   // PM2 will not output a combined file unless configured
  time: true
}]

--- PM2 report ----------------------------------------------------------------
Date                 : Mon Aug 12 2019 15:46:53 GMT-0700 (PDT)
===============================================================================
--- Daemon -------------------------------------------------
pm2d version         : 3.5.1
node version         : 8.15.1
node path            : /Users/brucejo/.nvm/versions/node/v8.15.1/lib/node_modules/pm2/bin/pm2
argv                 : /Users/brucejo/.nvm/versions/node/v8.15.1/bin/node,/Users/brucejo/.nvm/versions/node/v8.15.1/lib/node_modules/pm2/lib/Daemon.js
argv0                : node
user                 : undefined
uid                  : 0
gid                  : 0
uptime               : 4360min
===============================================================================
--- CLI ----------------------------------------------------
local pm2            : 3.5.1
node version         : 8.15.1
node path            : /Users/brucejo/.nvm/versions/node/v8.15.1/bin/pm2
argv                 : /Users/brucejo/.nvm/versions/node/v8.15.1/bin/node,/Users/brucejo/.nvm/versions/node/v8.15.1/bin/pm2,report
argv0                : node
user                 : brucejo
uid                  : 501
gid                  : 20
===============================================================================
--- System info --------------------------------------------
arch                 : x64
platform             : darwin
type                 : Darwin
cpus                 : Intel(R) Core(TM) i5-3210M CPU @ 2.50GHz
cpus nb              : 4
freemem              : 6981689344
totalmem             : 17179869184
home                 : /Users/brucejo
===============================================================================
--- PM2 list -----------------------------------------------
┌───────┬────┬──────┬────────┬───┬─────┬────────────┐
│ Name  │ id │ mode │ status │ ↺ │ cpu │ memory     │
├───────┼────┼──────┼────────┼───┼─────┼────────────┤
│ app   │ 12 │ fork │ online │ 0 │ 0%  │ 130.2 MB   │
│ meris │ 11 │ fork │ online │ 0 │ 0%  │ 129.1 MB   │
└───────┴────┴──────┴────────┴───┴─────┴────────────┘
Module
┌───────────────┬────┬────────┬─────┬────────────┐
│ Module        │ id │ status │ cpu │ mem        │
├───────────────┼────┼────────┼─────┼────────────┤
│ pm2-logrotate │ 0  │ online │ 0%  │ 104.0 MB   │
└───────────────┴────┴────────┴─────┴────────────┘
===============================================================================
--- Daemon logs --------------------------------------------
/Users/brucejo/.pm2/pm2.log last 20 lines:
PM2        | 2019-08-12T14:23:16: PM2 log: Stopping app:meris id:1
PM2        | 2019-08-12T14:23:16: PM2 log: Stopping app:app id:2
PM2        | 2019-08-12T14:23:16: PM2 log: App [app:2] exited with code [0] via signal [SIGINT]
PM2        | 2019-08-12T14:23:16: PM2 log: App [meris:1] exited with code [0] via signal [SIGINT]
PM2        | 2019-08-12T14:23:16: PM2 log: pid=80313 msg=process killed
PM2        | 2019-08-12T14:23:16: PM2 log: pid=80314 msg=process killed
PM2        | 2019-08-12T14:23:41: PM2 log: App [meris:9] starting in -fork mode-
PM2        | 2019-08-12T14:23:41: PM2 log: App [app:10] starting in -fork mode-
PM2        | 2019-08-12T14:23:41: PM2 log: App [meris:9] online
PM2        | 2019-08-12T14:23:41: PM2 log: App [app:10] online
PM2        | 2019-08-12T14:30:14: PM2 log: Stopping app:meris id:9
PM2        | 2019-08-12T14:30:14: PM2 log: Stopping app:app id:10
PM2        | 2019-08-12T14:30:14: PM2 log: App [meris:9] exited with code [0] via signal [SIGINT]
PM2        | 2019-08-12T14:30:14: PM2 log: App [app:10] exited with code [0] via signal [SIGINT]
PM2        | 2019-08-12T14:30:14: PM2 log: pid=76538 msg=process killed
PM2        | 2019-08-12T14:30:14: PM2 log: pid=76539 msg=process killed
PM2        | 2019-08-12T14:30:21: PM2 log: App [meris:11] starting in -fork mode-
PM2        | 2019-08-12T14:30:21: PM2 log: App [app:12] starting in -fork mode-
PM2        | 2019-08-12T14:30:21: PM2 log: App [meris:11] online
PM2        | 2019-08-12T14:30:21: PM2 log: App [app:12] online

Review the PM2 log-management documentation sections linked in the issue, especially ecosystem, disable logging, and CLI configuration. Update the explanation and example so the default -out and -err logs are distinguished from the separately configured combined log, then verify the documented behavior against the stated configuration.