[Propeller][ELF] Add Doxygen comment for decodeBBAddrMap

Adds a doxygen comment on decodeBBAddrMap specifying what it does as well
as some requirements that need to be met when calling it (especially
the condition of passing in a relocation section when the ELFFile is
relocatable).

Differential Revision: https://reviews.llvm.org/D145989