lintian: source-is-missing error for doxygen files

[Joan Lledó <jlledom@mailfence.com>]

Hello,

I'm maintaining the lwip package. The orig tarball includes doxygen
documentation, and this produces many "source-is-missing" lintian
errors. [1]

I don't know how to proceed to clean this errors, b/c I'm not sure
whether this is false-positive, an allowed exception or a bug on lintian.

I'd like to have your opinion on which would be the best approach to fix
this errors.

Thank you,

Joan.

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[1] https://lintian.debian.org/maintainer/jlledom@member.fsf.org.html#lwip