DOCS: updated documentation to revision 39.

Changes:
o Extended function! pointer description to struct members. Updated type casting matrix accordingly.
o Added function! as valid struct member.
o Added back the 'callback attribut description.
o Added 'custom attribut description.
o Added system/stack/align description.
o Added #call compiler directive description.
o Added #export compiler directive description.
o Added shared library and driver programs layouts sections.