This check tests that if a function has been declared deprecated
with either Q_DECL_DEPRECATED or has a @deprecated Doxygen note
then it MUST have both the Q_DECL_DEPRECATD and @deprecated note.
It's important that both are used, as Q_DECL_DEPRECATED allows
throwing a warning if that method is used in code, while the
@deprecated doxygen note gives an indication to devs/PyQGIS users
of why it's deprecated and what should be used instead.
Ideally we'd also test for SIP /Deprecated/ tags, but I can't
find any reliable way to do this.
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