notmuch(1): clarify documentation about --option/value separators

notmuch(1): clarify documentation about --option/value separators

id:CA+Tk8fzRiqxWpd=r8=DRvEewNZXUZgD7MKyRLB1A=R-LxxGEZw@mail.gmail.com
started a thread of discussion that showed that the cli's current
idiosyncrasies around dealing with boolean options were not
understandable.

This attempts to improve the documentation at least (actual changes to
the API might be better, but have not reached consensus).

Note that no one in the discussion thread identified any other
(non-boolean) command-line options that cannot use space as a
separator.  If such an option is identified (or introduced in the
future), it should be added explicitly to this part of the manual.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>