base: Add documentation to flags.hh

base: Add documentation to flags.hh

Add documentation to the Flags class. Use this opportunity
to rename some arguments to make their intention clearer.

Finally, the constructors have been merged using a default
value of 0.

Change-Id: I924b1d5c20a3e2066be64ab124ae1a5d96d4b3bf
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38735
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>