Clarify when hook_pathauto_pattern_alter() is called

Recently, I was confused when my implementation of hook_pathauto_pattern_alter() was not called for my new content type. I learned that hook_pathauto_pattern_alter() will only fire if a default pattern is defined. It makes sense to me that this hook would not fire if a default pattern is not defined, but I think it would be helpful to have a note in the API file that mentions this.