Restore parameter names in observable.h

Restore parameter names in observable.h

Ages ago, when we switched observables to be templates, Joel asked me
to restore the parameter names that were used in the old
observer.texi.

I've finally done this, putting the names into comments.  I also
updated the comments in this file to use the GNU metasyntactic
variable convention as well.

gdb/ChangeLog
2019-11-22  Tom Tromey  <tom@tromey.com>

	* observable.h: Update comments.

Change-Id: Id71bea7a7fcaa8f5d4491f33aa8861c56ba9c3f0