eina: Make return doxygen consistent

eina: Make return doxygen consistent

Summary:
Ensure @return defines error returns consistently.  In several cases the
errors were explained in the body but not mentioned in the @return docs.

Drop redundant return value documentation for clarity.  Some routines
were defining the return values both in @return and in the doxygen body.

Subscribers: cedric, jpeg

Differential Revision: https://phab.enlightenment.org/D5756