Minor "catch" documentation improvements

Minor "catch" documentation improvements

This patch makes a few minor improvements to the catchpoint
documentation:

* "catch exception" and "catch handlers" now mention the argument in
  the @item.

* "catch exception unhandled" is moved to be closer to "catch
  exception", rather than after "catch handlers".

* "catch load" and "catch unload" now wrap the argument in @var.

gdb/doc/ChangeLog
2019-05-10  Tom Tromey  <tromey@adacore.com>

	* gdb.texinfo (Set Catchpoints): Add text for parameter to "catch
	exception" and "catch handlers".  Move "catch exception unhandled"
	text.  Use @var for "catch load" and "catch unload"