Help: Add lib64, lib32, libx32 paths to find_package prefixes

Document the conditions when lib64, lib32, libx32 variants will be added to the list of possible installation prefixes.

Related source code:

• https://gitlab.kitware.com/cmake/cmake/blob/2572b824d4b81de83a1b16dee84d8754e846522c/Source/cmFindPackageCommand.cxx#L121-126
• https://gitlab.kitware.com/cmake/cmake/blob/2572b824d4b81de83a1b16dee84d8754e846522c/Source/cmFindPackageCommand.cxx#L1923-1925