FindPackageHandleStandardArgs: Clarify the FOUND_VAR option

The `find_package_handle_standard_args(FOUND_VAR)` is deprecated as of
CMake 3.3 and both the `<PackageName>_FOUND` and the upper-cased
`<PACKAGE_NAME>_FOUND` are set with or without using this option.
Additionally, find modules and docs are also synced with this to make it
clearer.