Improve comments in winhandl.c.

Improve comments in winhandl.c.

To understand the handle leak bug that I fixed in git commit
7549f2da40d3666f2c9527d84d9ed5468e231691, I had to think fairly hard
to remind myself what all this code was doing, which means the
comments weren't good enough. Expanded and rewritten some of them in
the hope that things will be clearer next time.

(cherry picked from commit a87a14ae0fc7d4621b5b1fafdb2053b3638b4b2b)

Cherry-picker's notes: this apparently pointless commit is required on
this branch because it's a dependency of the rather less pointless
9fec2e773873e28f1409f5e1eefaf03483070050.