Import all function documentations, return code reference
into the header files, as doc comments,
in the hopes of moving the CAref.html manual to docs.epics-controls.org,
which will be done in a later PR.
Also added doc comments to some other low-hanging fruits,
for example when plain comments where near, or when it was obvious to do so.
In some cases the license-identification header was missing,
so I added that as well. Replaced the remaining headers that
specifically identified "Versions 3.13.7 and higher".
Makefiles and the build system were deliberately excluded.
