Porting notes / deprecation docs

[Friedrich W. H. Kossebau]

Some hopefully helpful quick comments from couch:

Am Montag, 12. Juli 2021, 19:14:17 CEST schrieb Frederik Schwarzer:
> - If not documented separately, should existing deprecation messages
>    be improved? "no known users" might not be enough for the "unknown
>    users" in 3rd-party applications who get that message

Yes, ideally that should be backed up by some web page perhaps, informing 
anyone how to get in touch with whom to make a user and their needs known, for 
finding a solution.

> - Is it possible/desirable to keep the latest KF5 API docs as it is
>    generated on api.k.o to have deprecation messages with clickable links
>    to replacements?

When doing my own little contributions to keep api.kde.org alive last year, I 
also made sure to have the so far existing kdelibs 2-4 API still available, 
see https://api.kde.org/history.php (reached via "Old KDE Versions" from 
api.kde.org mainpage). The same hopefully can be done for KF5 (and other 
libraries who would need versioned docs).

Now what is meant by "clickable links to replacements" exactly? Any example 
for what you have in mind?
(Just in case, Doxygen usually itself already generated automatic links to the 
functions (just needs complete signature, incl. const), see also
https://www.doxygen.nl/manual/autolink.html
but then I would guess you know that)

Cheers
Friedrich