pg(1): add -p option

docs/pg.1

@@ -9,10 +9,23 @@
 
 pg \(en paginate
 
+.SH SYNOPSIS
+
+pg
+.RB ( -p
+.RB [ prompt ])
+
 .SH DESCRIPTION
 
 Pg prints standard input to standard output, accepting commands between pages.
 
+.SH OPTIONS
+
+.B -p
+.RS
+Replaces the default prompt (": ") with the next argument.
+.RE
+
 .SH DIAGNOSTICS
 
 Pg returns an unsuccessful exit code if the tty couldn't be opened or if pg was