help should be searchable

From a mailing list discussion:

Martin Pool writes:

  The length of 'bzr help commands', which was a concern to me previously
  about adding $verb-$subject commands, may be better handled by just adding
  some grouping in to that page, and having see-also links between commands.

Eli Zaretskii writes:

  Better yet, how about "bzr apropos REGEXP" and "bzr apropos-doc REGEXP"?

  The former matches against command names and the "Purpose" line, the
  latter matches against the whole documentation text of each command.

A single apropos command with a --doc parameter may be enough.