Driver: Replace a couple of out of date terms in the docs

The Job base class was removed in r241310, so replace a couple of
references to it with Command. Also change another use of Job with
Action, since that's the term used in the source.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241327 91177308-0d34-0410-b5e6-96231b3b80d8