doc: improve doc of g_file_equal()

doc: improve doc of g_file_equal()

A critical message is printed when a parameter of g_file_equal() is not
a GFile. When we read the documentation before this commit, we can think
that passing NULL or another type than GFile is allowed, but it is not
the case.

Another option is to allow NULL parameters. But for consistency with
e.g. g_str_equal(), it's probably better to keep the code as is.

https://bugzilla.gnome.org/show_bug.cgi?id=732357