Update some installation-related docs

- Add comments for installation-related variables in CMake, in
  particular the RPATH section that has caused some confusion.
- Document those few variables that can be used to alter the
  installation tree structure in the install guide.
- Reword some parts in the install guide about CMake iteration.
- Remove GMX_INSTALL_PREFIX, as there are other approaches to get the
  desired result with CPack that are less confusing.  This variable
  wasn't documented or probably used anywhere outside some dormant
  Jenkins jobs.

Change-Id: I199b06d905b45961313ed9fa44ae82e5987be0fe