[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