patch 9.1.0548: it's not possible to get a unique id for some vars

Problem:  it's not possible to get a unique id for some vars
Solution: Add the id() Vim script function, which returns a unique
          identifier for object, dict, list, job, blob or channel
          variables (Ernie Rael)

fixes: #14374
closes: #15145

Signed-off-by: Ernie Rael <errael@raelity.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 922e80a..765eab5 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -1408,6 +1408,8 @@
 
 	wordcount()		get byte/word/char count of buffer
 
+	id()			get unique string for item to use as a key
+
 	luaeval()		evaluate |Lua| expression
 	mzeval()		evaluate |MzScheme| expression
 	perleval()		evaluate Perl expression (|+perl|)