backfill: document acceptance of revision-range in more standard manner

backfill: document acceptance of revision-range in more standard manner

302aff09223f (backfill: accept revision arguments, 2026-03-26) added
support for passing revision arguments to 'git backfill' but documented
them only with a prose sentence:

    You may also specify the commit limiting options from
    git-rev-list(1).

No other command that accepts revision arguments documents them this
way.  Commands like log, shortlog, and replay define a formal
<revision-range> entry and include rev-list-options.adoc.  Commands like
bundle, fast-export, and filter-branch, which pass arguments through to
the revision machinery without including the full options file, still
define a formal <git-rev-list-args> entry explaining what is accepted.

Add a formal <revision-range> entry in the synopsis and OPTIONS section,
following the convention used by other commands, and mention that
commit-limiting options from git-rev-list(1) are also accepted.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>