gdb/doc: Update 'frame' command documentation.

gdb/doc: Update 'frame' command documentation.

The documentation for the 'frame' command has gotten a little out of
date, it still mentions architecturally specific details that are no
longer relevant.

This commit removes the old details that no longer apply, and tries to
expand the existing text a little to make the usage clearer for some
cases.

gdb/doc/ChangeLog:

	* gdb.texinfo (Selection): Update documentation for 'frame'
	command.