ozone: Note about PostCreateMainMessageLoop not called everywhere

In attempting to fix a test issue using crrev.com/c/6202104 it was found
that all tests may not call OzonePlatform::PostCreateMainMessageLoop().

Document this in the API for future reference.

R=fangzhoug, max@igalia.com

Change-Id: I2844aa681f7f806d87027fe7e83a728833fa1306
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6254059
Reviewed-by: Kramer Ge <fangzhoug@chromium.org>
Commit-Queue: Orko Garai <orko@igalia.com>
Reviewed-by: Max Ihlenfeldt <max@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1418848}