doc/Doxyfile: bump to version 1.8.3.1.

NB: @cond is borked.
Tidy; see full commit message.

Soft 80-column wrapping.

doc: pedantry, miscellanea.
doc: removed Doxygen @cond hack for ctor init lists with uniform initialization
(somehow Doxygen fixed this but broke @cond in the process).

Normalized template parameter names; pushed multi-parameter templates over
multiple lines.

Removed inline from member funcs (they are implicitly inline:
§7.1.2, para. 3).

Using plain unsigned and signed instead of unsigned int and int.

test: made main() parameterless (§3.6.1 para. 2).