docs: pdfdocs: Adjust \headheight for fancyhdr

Fancyhdr prior to v4.0 outputs a message per document as follows:

    Package Fancyhdr Warning: \headheight is too small (12.0pt):
    Make it at least 13.59999pt.
    We now make it that large for the rest of the document.
    This may cause the page layout to be inconsistent, however.

Fancyhdr v4.0 complains (once a page!) as follows:

    Package fancyhdr Warning: \headheight is too small (12.0pt):
    (fancyhdr)    Make it at least 13.59999pt, for example:
    (fancyhdr)    \setlength{\headheight}{13.59999pt}.
    (fancyhdr)    You might also make \topmargin smaller to compensate:

    (fancyhdr)    \addtolength{\topmargin}{-1.59999pt}.

Related item in fancyhdr v4.0 announcement on 2021-01-04 [1]:

    Backward incompatible changes:
      - Eliminate adjustments of \headheight or \footskip, when the
        header or footer is too high.

[1]: https://www.ctan.org/ctan-ann/id/[email protected]

Silence the warnings by adding a couple of \addtolength commands in
the preamble.

Signed-off-by: Akira Yokosawa <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>

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