Fixed GNUmakefile to use appropriate default executable.  Made small documentation fixes.

Documentation fixes in books/system/doc/acl2-doc.lisp:

- Fixed typos "sould" --> "should" found by Eric Smith.

- Fixed parent of hons-enabled.

- Improved :doc linear (thanks to feedback from David Rager).

- Added :doc guard-holders and referenced it from other topics.

Quoting :doc note-7-0:

  For Makefile targets for testing such as regression, the default ACL2
  image used is now the same one that would be built for the same
  Makefile variables. In particular, the command make regression is
  equivalent to make regression ACL2_HONS=h, which uses the
  executable saved_acl2h, not saved_acl2 as was formerly the case.
  Thanks to Harsh Raju Chamarthi for finding a bug in a first attempt
  to make this change.

Also fixed banner to avoid saying "experimental" for ACL2(h).
Similarly fixed some comments.

Fixed memoize.lisp header to avoid implying that memoize.lisp is not
included in ACL2(c) (i.e., ACL2 that is not hons-enabled), since that
has been false for some time.