Add suggestion for lua headers inclusion issues

On some systems "latests" lua header files are located in /usr/include, while the header files for older lua versions can be found in subdirectories such as /usr/include/lua5.1 or similar.

In those situations, despite compiling for an older lua version, CMake will include the headers from /usr/include resulting in a broken build only detectable at runtime. (see https://gist.github.com/FunkyAss/c2a8095a2317bb69fd73).

As stated in the commit message, patching FindLua*.cmake files is pointless, so adding a note in the documentation should be fine.