[docs][llvm-ar] Update llvm-ar command guide

The llvm-ar command guide had not been updated in some time, it was
missing current functionality and contained information that was out
of date. This change:
- Updates the use of reStructuredText directives, as seen in other tools
  command guides.
- Updates the command synopsis.
- Updates the descriptions of the tool behaviour.
- Updates the options section.
- Adds details of MRI script functionality.
- Removes the sections "Standards" and "File Format"

Differential Revision: https://reviews.llvm.org/D68998

llvm-svn: 375412