[PM] Add proper documentation for the ModulePassManager and

FunctionPassManager. These never got documented, likely due to the
clutter of this header file. This fixes another problem people noticed
when they started trying to use the new pass manager.

I've also used this to document the aspirational constraints I would
like to hold passes to. I don't really have a better place to document
such things at this point, but eventually will probably create a proper
.rst file and page for the LLVM pass infrastructure that carries such
high-level concerns.

llvm-svn: 225097