CXX-1303 Amend supported compiler docs
CXX-1303 Amend supported compiler docs

This simplifies the text of the "older than tested" paragraph so
readers don't miss the main point.  It also adds a link to JIRA
for bug reports.

xdg/mongo-cxx-driver,acmorrow/mongo-cxx-driver,xdg/mongo-cxx-driver,acmorrow/mongo-cxx-driver,mongodb/mongo-cxx-driver,xdg/mongo-cxx-driver,xdg/mongo-cxx-driver,acmorrow/mongo-cxx-driver,mongodb/mongo-cxx-driver,acmorrow/mongo-cxx-driver,mongodb/mongo-cxx-driver,mongodb/mongo-cxx-driver