git-gc: improve wording of --auto notification

The previous message had too much of a "boy, you should
really turn off this annoying gc" flair to it. Instead,
let's make sure the user understands what is happening, that
they can run it themselves, and where to find more info.

Suggested by Brian Gernhardt.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

builtin-gc.c

@@ -206,9 +206,9 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 		if (!need_to_gc())
 			return 0;
 		fprintf(stderr, "Packing your repository for optimum "
-			"performance. If you would rather run\n"
-			"\"git gc\" by hand, run \"git config gc.auto 0\" "
-			"to disable automatic cleanup.\n");
+			"performance. You may also\n"
+			"run \"git gc\" manually. See "
+			"\"git help gc\" for more information.\n");
 	} else {
 		/*
 		 * Use safer (for shared repos) "-A" option to