Enhance add-extra-docs! error messages to distinguish some cases

such as the difference between:

1. The case when there are extra docs in the resource file for a
   namespace, but the user does not have such a namespace loaded at
   all.  Only warn once per namespace, not per symbol in that
   namespace.

2. The case when the user has a namespace loaded, but a var is found
   in the extra docs resource file that is not in that namespace as
   the user has curently loaded it.