patch 9.0.1688: cannot store custom data in quickfix list

Problem: cannot store custom data in quickfix list
Solution: add `user_data` field for the quickfix list

closes: #11818

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Tom Praschan <13141438+tom-anders@users.noreply.github.com>
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index e74b397..979f46f 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -4002,6 +4002,9 @@
 			text	description of the error
 			type	type of the error, 'E', '1', etc.
 			valid	|TRUE|: recognized error message
+		        user_data
+			        custom data associated with the item, can be
+				any type.
 
 		When there is no error list or it's empty, an empty list is
 		returned. Quickfix list entries with a non-existing buffer
@@ -8298,6 +8301,8 @@
 		    text	description of the error
 		    type	single-character error type, 'E', 'W', etc.
 		    valid	recognized error message
+		    user_data   custom data associated with the item, can be
+				any type.
 
 		The "col", "vcol", "nr", "type" and "text" entries are
 		optional.  Either "lnum" or "pattern" entry can be used to