Improve documentation around TS_execute().

Improve documentation around TS_execute().

I got frustrated by the lack of commentary in this area, so here is some
reverse-engineered documentation, along with minor stylistic cleanup.
No code changes more significant than removal of unused variables.

Back-patch to 9.6, not because that's useful in itself, but because
we have some bugs to fix in phrase search and this would cause merge
failures if it's only in HEAD.