man/ostree: Document --repo option a bit more

This new information is already mostly part of `ostree.repo(5)`, though
let's put it in `ostree(1)` as well since that's where the switch is
officially documented.

Closes: #1560
Approved by: cgwalters

src/ostree/ot-main.c

@@ -46,7 +46,7 @@ static GOptionEntry global_entries[] = {
 };
 
 static GOptionEntry repo_entry[] = {
-  { "repo", 0, 0, G_OPTION_ARG_FILENAME, &opt_repo, "Path to OSTree repository (defaults to /sysroot/ostree/repo)", "PATH" },
+  { "repo", 0, 0, G_OPTION_ARG_FILENAME, &opt_repo, "Path to OSTree repository (defaults to current directory or /sysroot/ostree/repo)", "PATH" },
   { NULL }
 };