Documentation files should have license notices also. Manuals should use the GNU Free Documentation License. Following is an example of the license notice to use after the copyright line(s) using all the features of the GFDL.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being ``GNU General Public License'', 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 the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom.''
If the FSF does not publish this manual on paper, then omit the last sentence in (a) that talks about copies from GNU Press. If the FSF is not the copyright holder, then replace ‘FSF’ with the appropriate name.
Please adjust the list of invariant sections as appropriate for your manual. If there are none, then say “with no Invariant Sections”. If your manual is not published by the FSF, and under 400 pages, you can omit both cover texts. However, if it is copyright FSF, always ask the FSF what to do.
See GNU Sample Texts in Texinfo, for a full example in a Texinfo manual, and see https://www.gnu.org/licenses/fdl-howto.html for more advice about how to use the GNU FDL.
If you write a manual that people might want to buy on paper, please write to firstname.lastname@example.org to tell the FSF about it. We might want to publish it.
If the manual is over 400 pages, or if the FSF thinks it might be a good choice for publishing on paper, then please include the GNU GPL, as in the notice above. Please also include our standard invariant section which explains the importance of free documentation. Write to email@example.com to get a copy of this section.
When you distribute several manuals together in one software package, their on-line forms can share a single copy of the GFDL (see section 6). However, the printed (‘.dvi’, ‘.pdf’, …) forms should each contain a copy of the GFDL, unless they are set up to be printed and published only together. Therefore, it is usually simplest to include the GFDL in each manual.