Update documentation about convergent attribute.

Summary:
Be more explicit about what 'convergent' means, and indicate that the
compiler may remove the attribute from a function if it can prove that
the function doesn't in fact execute any convergent ops.

Reviewers: resistor, jingyue, joker.eph

Subscribers: hfinkel, chandlerc, arsenm, jhen, tra, llvm-commits

Differential Revision: http://reviews.llvm.org/D17012