[llvm-objdump][docs] Add details to the help output and command guide

This change is to add some missing details, clarifies some options and
brings the help text and command guide of objdump closer together.

- Added to the help that --all-headers also outputs symbols and
  relocations to match the command guide.
- Added to the help that --debug-vars accepts an optional
  ascii/unicode format to match the command guide.
- Changed the help descriptions for --disassemble,
  --disassemble-all, --dwarf=<value>, --fault-map-section,
  --line-numbers, --no-leading-addr and --source descriptions to
  match the command guide.
- Added to the help that --start-address and --stop-address also
  effect relocation entries and the symbol table output to match
  the command guide.
- Added a note to the command guide that --unwind-info and -u
  are not available for the elf format.

Differential Revision: https://reviews.llvm.org/D110633