[docs] cleanup documentation on lit substitutions

1. Added missing substitutions to the documentation in docs/TestingGuide.rst
2. Modified docs/CommandGuide/lit.rst to only document the "base" set of substitutions and to refer the reader to docs/TestingGuide.rst for more detailed info on substitutions.

Patch by bd1976llvm

Differential Revision: https://reviews.llvm.org/D29281

llvm-svn: 294586