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

302aff0 (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>

Author: newren

Documentation/git-backfill.adoc

@@ -9,7 +9,7 @@ git-backfill - Download missing objects in a partial clone
 SYNOPSIS
 --------
 [synopsis]
-git backfill [--min-batch-size=<n>] [--[no-]sparse]
+git backfill [--min-batch-size=<n>] [--[no-]sparse] [<revision-range>]
 
 DESCRIPTION
 -----------
@@ -43,7 +43,7 @@ smaller network calls than downloading the entire repository at clone
 time.
 
 By default, `git backfill` downloads all blobs reachable from the `HEAD`
-commit. This set can be restricted or expanded using various options.
+commit. This set can be restricted or expanded using various options below.
 
 THIS COMMAND IS EXPERIMENTAL. ITS BEHAVIOR MAY CHANGE IN THE FUTURE.
 
@@ -63,7 +63,16 @@ OPTIONS
 	current sparse-checkout. If the sparse-checkout feature is enabled,
 	then `--sparse` is assumed and can be disabled with `--no-sparse`.
 
-You may also specify the commit limiting options from linkgit:git-rev-list[1].
+`<revision-range>`::
+	Backfill only blobs reachable from commits in the specified
+	revision range.  When no _<revision-range>_ is specified, it
+	defaults to `HEAD` (i.e. the whole history leading to the
+	current commit).  For a complete list of ways to spell
+	_<revision-range>_, see the "Specifying Ranges" section of
+	linkgit:gitrevisions[7].
++
+You may also use commit-limiting options understood by
+linkgit:git-rev-list[1] such as `--first-parent`, `--since`, or pathspecs.
 
 SEE ALSO
 --------

builtin/backfill.c

@@ -26,7 +26,7 @@
 #include "path-walk.h"
 
 static const char * const builtin_backfill_usage[] = {
-	N_("git backfill [--min-batch-size=<n>] [--[no-]sparse]"),
+	N_("git backfill [--min-batch-size=<n>] [--[no-]sparse] [<revision-range>]"),
 	NULL
 };