docs: Clarify >=2.76 changes to g_module_open()

It is not clear in the documentation for gmodule >= 2.76 that the behaviour of g_module_open{,_full} has changed. Previously it did not handle prefixes without using g_module_build_path(), so something like g_module_open ("/some/lib/path/my-cool-module"); will work on >= 2.76 if, eg, /some/lib/path/libmy-cool-module.so exists, but this is not the case on < 2.76.

IMO, this change of behaviour should be documented so developers can structure their code accordingly.