Updated Doxyfile to 1.8.7.
doc: make section names unique.
doc: strict ordering of sections in detailed documentation.

It makes sense, but this ordering also helps to avoid bug in Doxygen
1.8.6+ which merges next non-xrefitem section with the previous one (so
e.g. the TODO list contained the following @see block, which is not
desired).
Doxygen workarounds.
doc: removed old Doxygen workarounds.

It can finally handle operators, yay! (Well, some of them...)