Refer to 'mask' instead of 'block' in Control.Exception

Summary: More thorough version of a75383cd

Test Plan:
change of comments only

[skip ci]

Reviewers: austin, simonmar, ekmett

Reviewed By: austin, ekmett

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D239