Improved :doc set-gag-mode a bit.

Quoting :doc note-6-4:

| Gag-mode no longer saves prover output when PROVE output is
| inhibited (see :DOC SET-INHIBIT-OUTPUT-LST).  This may improve performance
| slightly when certifying community books using their Makefile or
| cert.pl; in particular, GCL 2.6.8 running on a Mac can now certify the
| book books/tau/bounders/elementary-bounders.lisp without running out of
| space, even without explicitly turning off gag-mode (which had been done
| shortly before the Version  6.3 release).

| When include-book fails to find a readable certificate (.cert)
| file, the error message now distinguishes between the case that this file is
| missing and the case that read permission is missing.


git-svn-id: http://acl2-devel.googlecode.com/svn/trunk@1116 5d84a466-a78b-0930-a311-59b22171a541