Documentation license

The doc/gengetopt.texinfo file contains the following comment:

@c GNU gengetopt is free software; you can redistribute it and/or modify
@c it under the terms of the GNU General Public License as published by
@c the Free Software Foundation; either version 3, or (at your option)
@c any later version.


But later the same file reads:

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
and with the Back-Cover Texts as in (a) below.  A copy of the
license is included in the section entitled ``GNU Free Documentation
License.''

(a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify
this GNU Manual, like GNU software.  Copies published by the Free
Software Foundation raise funds for GNU development.''
@end quotation


Could you please clarify how is the documentation licensed?
Do I need to follow conditions of both licenses? Either of them? One 
of them? (Which?)

Thanks!

-- 
Jakub Wilk