diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index fb8b116..53179ca 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -195,7 +195,7 @@
 foldtext()			String	line displayed for closed fold
 foldtextresult({lnum})		String	text for closed fold at {lnum}
 foreground()			Number	bring the Vim window to the foreground
-fullcommand({name})		String	get full command from {name}
+fullcommand({name} [, {vim9}])	String	get full command from {name}
 funcref({name} [, {arglist}] [, {dict}])
 				Funcref	reference to function {name}
 function({name} [, {arglist}] [, {dict}])
@@ -2967,14 +2967,20 @@
 		{only in the Win32, Motif and GTK GUI versions and the
 		Win32 console version}
 
-fullcommand({name})						*fullcommand()*
+fullcommand({name} [, {vim9}])				*fullcommand()*
 		Get the full command name from a short abbreviated command
 		name; see |20.2| for details on command abbreviations.
 
 		The string argument {name} may start with a `:` and can
 		include a [range], these are skipped and not returned.
-		Returns an empty string if a command doesn't exist or if it's
-		ambiguous (for user-defined commands).
+		Returns an empty string if a command doesn't exist, if it's
+		ambiguous (for user-defined commands) or cannot be shortened
+		this way. |vim9-no-shorten|
+
+		Without the {vim9} argument uses the current script version.
+		If {vim9} is present and FALSE then legacy script rules are
+		used.  When {vim9} is present and TRUE then Vim9 rules are
+		used, e.g. "en" is not a short form of "endif".
 
 		For example `fullcommand('s')`, `fullcommand('sub')`,
 		`fullcommand(':%substitute')` all return "substitute".