GMP docs not shipped "due to licensing"

[Simon Sobisch]

gmp/debian/README.Debian says

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Sadly, due to licensing the GMP documentation is no longer shipped in
Debian.  The package libgmp10-doc now contains only demo code.
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and links to the manual which says:

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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 no 
Invariant Sections, with the Front-Cover Texts being “A GNU Manual”, and 
with the Back-Cover Texts being “You have freedom to copy and modify 
this GNU Manual, like GNU software”.
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Can you please share where the licensing issue is? I don't think that's 
the GFDL - because this manual has no invariant sections - is it?

Thank you,
Simon