Be more descriptive warning when licenses is empty

Previously RubyGems said "licenses is empty" without giving any further
guidance.  This didn't help users adjust to the new warning.

Now rubygems points to the OSI license list so users have a better idea
of what to do to silence the warning.

Fixes #656
Include license list in Specification#license=

This follows #656, and gives better guidance to users setting licenses.
Improve Specification#license documentation

Now the various license methods link to #license= where the list lives.