Refer to -help instead of --help since this is what tools themselves say.

Refer to -help instead of --help since this is what tools themselves say.
Also, have tools output -help-hidden rather than refer to --help-hidden,
for consistency, and likewise adjust documentation.  This doesn't change
every mention of --help, only those which seemed clearly safe.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@96578 91177308-0d34-0410-b5e6-96231b3b80d8