Improving the comments in pqsignal()

Hi,

While following along with Tristan and Heikki's thread about signals
in psql, it occurred to me that the documentation atop pqsignal() is
not very good:

 * we don't explain what problem it originally solved
 * we don't explain why it's still needed today
 * we don't explain what else it does for us today
 * we describe the backend implementation for Windows incorrectly (mea culpa)
 * we vaguely mention one issue with Windows frontend code, but I
think the point made is misleading, and we don't convey the scale of
the differences

Here is my attempt to improve it.