Fix help and documentation for inferior commands

Fix help and documentation for inferior commands

This changes inferior.c to add Usage lines for all commands, and to
change how "metasyntactic variables" are written to conform to GNU
style.

While doing this I noticed that the manual doesn't document the
argument to "info inferiors", so I've added that as well.

ChangeLog
2018-04-27  Tom Tromey  <tom@tromey.com>

	* inferior.c (initialize_inferiors): Update help strings.

doc/ChangeLog
2018-04-27  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Inferiors and Programs): Document argument to "info
	inferiors".