Documentation issue: When do binary runfiles get invalidated by `bazel build`, etc?

### Description of the problem / feature request:

With help from colleagues, I've built an internal model of when to expect my runfiles to become stale (e.g. should be rebuilt) and/or invalidated (rebuilt due to an update / action env change / etc.).

As I look through the Bazel docs for 4.0.0, I can't seem to find a brief summary of this setup.

(It could be I was looking for the wrong thing / didn't look in enough places?)

Closest I got was stumbling around w/ search and clicking on the wesbite and coming across stuff like this:
https://docs.bazel.build/versions/4.0.0/skylark/rules.html#runfiles

### Feature requests: what underlying problem are you trying to solve with this feature?

Better docs!

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Docs! Couldn't easily find a place where this is. (Maybe I didn't search correctly?)

### What operating system are you running Bazel on?

Ubuntu 18.04.5

### What's the output of `bazel info release`?

`release 4.0.0`

### If `bazel info release` returns "development version" or "(@non-git)", tell us how you built Bazel.

N/A

### What's the output of `git remote get-url origin ; git rev-parse master ; git rev-parse HEAD` ?

https://github.com/RobotLocomotion/drake
693c8497ff4d8bb7869ccbf0a9d8fbe90aa8c92d
693c8497ff4d8bb7869ccbf0a9d8fbe90aa8c92d

### Have you found anything relevant by searching the web?

Searched for `runfiles invalidation`, but didn't see much aside from the above doc link, and stuff like this:

- https://docs.bazel.build/versions/4.0.0/glossary.html#runfiles
- https://docs.bazel.build/versions/4.0.0/glossary.html#action-key
- https://docs.bazel.build/versions/4.0.0/output_directories.html
- https://docs.bazel.build/versions/4.0.0/user-manual.html#flag--workspace_status_command

### Any other information, logs, or outputs that you want to share?

Nah. But thanks for asking!

Start by reading the linked runfiles, glossary, output_directories, and user-manual documentation for Bazel 4.0.0, focusing on action keys and runfile invalidation. Document a brief, findable explanation of when binary runfiles become stale or are rebuilt, with the relevant existing references linked.

Written by the indexing model from the issue text.