backfill: mark it as experimental

This is a highly useful command, and we want it to get some testing "in
the wild". However, the patches have not yet been reviewed on the Git
mailing list, and are therefore subject to change. By marking the
command as experimental, users will be warned to pay attention to those
changes.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/git-backfill.txt

@@ -9,7 +9,7 @@ git-backfill - Download missing objects in a partial clone
 SYNOPSIS
 --------
 [verse]
-'git backfill' [--batch-size=<n>] [--[no-]sparse]
+(EXPERIMENTAL) 'git backfill' [--batch-size=<n>] [--[no-]sparse]
 
 DESCRIPTION
 -----------

builtin/backfill.c

@@ -25,7 +25,7 @@
 #include "path-walk.h"
 
 static const char * const builtin_backfill_usage[] = {
-	N_("git backfill [--batch-size=<n>] [--[no-]sparse]"),
+	N_("(EXPERIMENTAL) git backfill [--batch-size=<n>] [--[no-]sparse]"),
 	NULL
 };