Fix documentation for $wgResourceLoaderLESSVars

* These aren't string values, but LESS code snippets. This rarely
  matters, but it matters very much when it does.
* Changes to the values do not actually trigger cache invalidation.
  Hashed contents are only used for "second-level" cache keys and do
  not make it possible to invalidate module cache.

Change-Id: I373dfedac1e43db4d0aa709384eee7f7e7f1808c