Enable chpldoc extern flag?

Open question: would we want to give the ability for a chpldoc user to avoid outputting extern declarations via a flag? The support is mostly present today and theoretically only involves uncommenting this line to complete but we aren't sure of the utility.

This was considered because extern functions are often used for under-the-hood code that is generally not something we want to expose to clients of the particular module, and so might fall under the same category as private functions.

Compile command: chpldoc externTest.doc.chpl

Associated Future Test(s): test/chpldoc/compflags/extern/externTest.doc.chpl