Mention capping forbid lints

I felt the description of forbid was misleading/incomplete without
mentioning how --cap-lints interacts with it.

src/doc/rustc/src/lints/levels.md

@@ -90,7 +90,9 @@ This lint level gives you that.
 'forbid' is a special lint level that's stronger than 'deny'. It's the same
 as 'deny' in that a lint at this level will produce an error, but unlike the
 'deny' level, the 'forbid' level can not be overridden to be anything lower
-than an error.
+than an error.  However, lint levels may still be capped with `--cap-lints`
+(see below) so `rustc --cap-lints warn` will make lints set to 'forbid' just
+warn.
 
 ## Configuring warning levels