Fix most Doxygen 1.8.16 warnings in optimizer code. The most common
new warning is having documentation on both declaration and definition
of the same function, which is not allowed (and unsurprisingly, a lot
of them had diverged in content). Fix by moving the one that looked
the most up-to-date to the definition, which is where everyone who
actually needs the documentation will be looking.

There's one mystery case left in json_path.h, which I hope someone
else will figure out.

Change-Id: Ia00655c9b5f8e489360c6bbaa02122e6324dc621