[llvm-objdump] Update the doc for --disassemble-functions.

Update the doc after llvm-svn: 364121 is landed.
With two more trivial fixes that are not related to
--disassemble-functions but still about llvm-objdump.

Reviewers: jhenderson, grimar, MaskRay, rupprecht, peter.smith

Reviewed by: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D63787

llvm-svn: 364573