Add role weight to documentation

Took me an hour or so to find out what the effect of role weight could be on the path access. I suggest to add a line of comment on this in path_access_help:

For users with more than one role, the role weight determines which role is used for access. The role with the highest weight number is used, other roles are ignored.

This is my first time contributing, perhaps this is not the proper way of doing it. If not, let me know.