misc: Update documentation of SimObject related APIs

misc: Update documentation of SimObject related APIs

Updated documentation of Drain, Serialize, Evnet queue and Simobject
APIs. Made some corrections to where the documentation was available
in the code but did not appear in the documentation.

Change-Id: I5254e87eb5663232e824bcd5592da0a04eba673b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31814
Reviewed-by: Ayaz Akram <yazakram@ucdavis.edu>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>