[docs] Improvements to CMake.rst

- Fix some grammatical and typographical errors.
- Try to improve upon some awkward/nonstandard phrasings.
- Expand slightly the treatment of how you specify arguments to cmake.
- Update the list of possible LLVM_BUILD_TESTS and state where to find the
  definitive list.
- Correct the name of llvm-tblgen.
- Expand slightly the treatment of several build options, including
  LLVM_LIT_TOOLS_DIR, LLVM_ENABLE_FFI, and LLVM_EXTERNAL_project_SOURCE_DIR.

Patch by Brian R. Gaeke!

Differential Revision: http://reviews.llvm.org/D11862

llvm-svn: 245911