Suggested by G. Brandon Robinson.

The overloading of IR emits magic \X'...' sequences that turn into HTML manual links. But not all such IR invocations should be manual links; those had to be written to avoid the IR macro before. Worse, the \X'...' ending the IR causes troff to emit only a single space after a period. Defining a new IM macro for manual references fixes both problems. Fixes #441.

Fixes #357

Change-Id: I940a21d67b21b5dd139cb20b9fae9595506fc5ae

LGTM=rsc R=rsc https://codereview.appspot.com/71070050

R=rsc CC=codebot http://codereview.appspot.com/637041