Clarify use_decl module resolution docs.

Attempt #2 of [this pull request](https://github.com/mozilla/rust/pull/8597) since my tired mind decided messing up my repo, then deleting and re-forking was a good idea.  This is identical to the second last commit in that PR.  (The one before I ruined everything.)

This is an improvement to use_decl docs, which will hopefully improve understanding of how resolution works and where `extern mod` and module declarations should go to make them visible to use declarations.