Removed legacy documentation support from ACL2 sources, and correspondingly modified documentation and a few books.  More below:

Added to :doc note-7-0:

  We removed support for processing legacy [documentation] strings
  (those starting with ":Doc-Section"), since the [xdoc] system has
  been stable for some time. Thanks to Jared Davis for assisting in
  that effort and to David Rager for his encouragement. The ACL2
  system and [community-books] still have code to support such
  processing, but it is essentially commented out: specifically, each
  such piece of code is prefixed by #+acl2-legacy-doc. We expect to
  delete such code entirely before the next release.

Deleted from :doc note-7-0 (obsolete given what's above, plus
``Tours'' topics seem OK as far as I can tell, in the web-based
manual):

  We removed support for legacy [documentation] topics
  SWITCHES-PARAMETERS-AND-MODES and the ``Tours''. (The
  implementation-level change was to remove such entries from source
  function documentation-alist-stub.) While the community-books and
  ACL2-only documentation continues to be maintained, existing
  old-style (pre-[xdoc]) documentation topics that use those deleted
  topics as a parent will likely need to have a different parent.