Pushed, with some fooling with the documentation (notably, re-alphabetizing relevant lists).
Grepping turned up an additional place that's examining the PAGER environment variable, namely PQprint() in libpq/fe-print.c. That's not used by psql (or anything else in core PG) anymore; we're only keeping it around for hypothetical external users of libpq. I debated whether to make it honor PSQL_PAGER. I decided not to, since those external users are by definition not psql ... but I added a comment about that.