pgsql: Improve comments about pqsignal().

Improve comments about pqsignal().

Explain where pqsignal() came from, what problem it originally solved
without assuming the reader is familiar with historical Unixen, why we
still need it, what it does for us now, and the key differences in
frontend code on Windows.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CA%2BhUKG%2BRst1h3uo%2BXRgdRVnWHBa4mmj5gFbmCzZr73s-Fh_5JA%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/262454be9a62d69d7523c0c88c7d07d5971a0fa0

Modified Files
--------------
src/port/pqsignal.c | 34 ++++++++++++++++++++++++++--------
1 file changed, 26 insertions(+), 8 deletions(-)