Refer readers of \? to "\? variables" for pset options

... and remove the list of \pset options from the general \? output.
That list was getting out of hand, both for developers to keep up to
date as well as for users to read.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/202511041638.dm4qukcxfjto@alvherre.pgsql

src/bin/psql/help.c

@@ -290,12 +290,7 @@ slashUsage(unsigned short int pager)
 	HELPN("  \\H                     toggle HTML output mode (currently %s)\n",
 		  ON(pset.popt.topt.format == PRINT_HTML));
 	HELP0("  \\pset [NAME [VALUE]]   set table output option\n"
-		  "                         (border|columns|csv_fieldsep|display_false|display_true|\n"
-		  "                         expanded|fieldsep|fieldsep_zero|footer|format|linestyle|\n"
-		  "                         null|numericlocale|pager|pager_min_lines|recordsep|\n"
-		  "                         recordsep_zero|tableattr|title|tuples_only|\n"
-		  "                         unicode_border_linestyle|unicode_column_linestyle|\n"
-		  "                         unicode_header_linestyle|xheader_width)\n");
+		  "                         see \"\\? variables\" for valid options\n");
 	HELPN("  \\t [on|off]            show only rows (currently %s)\n",
 		  ON(pset.popt.topt.tuples_only));
 	HELP0("  \\T [STRING]            set HTML <table> tag attributes, or unset if none\n");