Re-think our doxygen warnings wrt missing docs and fatalness.

Up till now, we have warned about all missing documentation, which
meant that we could never make doxygen warnings fatal.  This has led
to our doxygen output getting full of errors several times in the
past.

This commit changes our approach to doxygen warnings: missing
documentation warnings are only on when the user asks for them with
--enable-missing-doc-warnings.  When that option is not present,
doxygen respects the --enable-fatal-warnings flag.

Closes ticket 32385.