+ printf(_(" ECHO_HIDDEN display internal queries executed by backslash commands\n"));
I think "(same as ...)" can be omitted to keep the description short. If you want to retain it, other variables should also accompany similar description, such as -a for ECHO.
(5) + printf(_(" FETCH_COUNT fetch many rows at a time (use less memory) (default 0 unlimited)\n"));
should better be:
+ printf(_(" FETCH_COUNT the number of result rows to fetch and display at a time (default: 0=unlimited)\n"));
(6) + printf(_(" HISTCONTROL when set, controls history list [ignorespace, ignoredups, ignoreboth]\n"));
should better be:
+ printf(_(" HISTCONTROL control history list [ignorespace, ignoredups, ignoreboth]\n"));
(7) + printf(_(" USER the database user currently connected\n"));
should add "as" at the end:
+ printf(_(" USER the database user currently connected as\n"));
(8) "Printing options" section lack the following ones described in psql manual: