ueventd: update the comment on parallel handling

ueventd.cpp has comments on the parallel handling of uevent messages,
but it has two points to fix.
1. Semantics of setegid and setfscreatecon have not been updated after
   we added ueventd_test.cpp that continuously confirms that those
   functions have per-thread semantics, not per-process semantics.
2. It assumes uevent messages can be completely handled in isolation,
   but this has some nuance about event dependency.

This change fixes these points.

Bug: 400592897
Test: build succeeds
Change-Id: Ic8cf669c2b002900841f7d8656348cbe4652a726