SI-6812 scaladoc can opt out of expanding macros

This is a temporary change, possible only because macros currently can't
affect the global symbol table (except for the case when they will steer
inference of a method's return type).

Later on, e.g. with the addition of c.introduceTopLevel in master,
we will have to upgrade Scaladoc to allow for separate generation of
documentation, because then we'll be forced to expand macros in order to
get the whole picture of the code.
Merge pull request #2024 from scalamacros/ticket/6812

SI-6812 scaladoc can opt out of expanding macros