psql: Include \pset xheader_width in --help=commands|variables

psql's --help was missed the description of the \pset variable
xheader_width, that should be listed when using \? or --help=commands,
and described for --help=variables.

Oversight in a45388d6e0.

Author: Pavel Luzanov
Discussion: https://postgr.es/m/1e3e06d6-0807-4e62-a9f6-c11481e6eb10@postgrespro.ru
Backpatch-through: 16

src/bin/psql/help.c

@@ -317,7 +317,7 @@ slashUsage(unsigned short int pager)
 		  "                         numericlocale|pager|pager_min_lines|recordsep|\n"
 		  "                         recordsep_zero|tableattr|title|tuples_only|\n"
 		  "                         unicode_border_linestyle|unicode_column_linestyle|\n"
-		  "                         unicode_header_linestyle)\n");
+		  "                         unicode_header_linestyle|xheader_width)\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");
@@ -517,6 +517,9 @@ helpVariables(unsigned short int pager)
 		  "  unicode_column_linestyle\n"
 		  "  unicode_header_linestyle\n"
 		  "    set the style of Unicode line drawing [single, double]\n");
+	HELP0("  xheader_width\n"
+		  "    set the maximum width of the header for expanded output\n"
+		  "    [full, column, page, integer value]\n");
 
 	HELP0("\nEnvironment variables:\n");
 	HELP0("Usage:\n");