parser.cc: Logic for WarningsPolicy() about multiple unallowed root-level elements only applies for //include models?

## Environment
* OS Version: Ubuntu 18.04
* Source build, `sdformat11_11.1.0`

## Description
* Expected behavior: `WarningsPolicy()` allows us to configure multiple unallowed root-level elements for any model (direct parsing or via `//include`).
* Actual behavior: It only seems to be emitted via `//include` logic.

## Steps to reproduce
Per investigation here: https://github.com/RobotLocomotion/drake/pull/15075#pullrequestreview-675795003

See:
https://github.com/osrf/sdformat/blob/bed1589b0a29eaacc556ab0b76fa04c58ebd107f/src/parser.cc#L1185-L1214

## Output
N/A