Document CMAKE_INSTALL_PREFIX in CMAKE_SYSTEM_PREFIX_PATH

Add documentation to both variables to indicate that the install prefix
is also searched.
Merge topic 'doc-search-install-prefix' into next

a0b37d7 Document CMAKE_INSTALL_PREFIX in CMAKE_SYSTEM_PREFIX_PATH
Add cmake_host_system_information command

Expose the internal system information API to the CMake language.  For
example, it is useful to see how much memory the system has available to
estimate an upper limit of tests that can run in parallel.
Merge topic 'add-cmake_host_system_information-command' into next

7a9f393 Add cmake_host_system_information command