Update help text in ada-lang.c

I noticed that the "catch assert" help text erroneously claimed to
accept an argument, and while fixing this I went ahead and added
"Usage" text and made other minor updates to the commands in
ada-lang.c.

Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-06-10  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (_initialize_ada_language): Update help text.