[gio] Improve doc for g_file_make_directory_with_parents()
[gio] Improve doc for g_file_make_directory_with_parents()

g_file_make_directory_with_parents() will fail for already
existing directories, unlike g_mkdir_with_parents(), so mention
this clearly in the docs.

https://bugzilla.gnome.org/show_bug.cgi?id=660791

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