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

[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missing current functionality and contained information that was out
of date.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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375412 91177308-0d34-0410-b5e6-96231b3b80d8