patch 8.2.1771: synIDattr() cannot get the value of ctermul

Problem:    synIDattr() cannot get the value of ctermul.
Solution:   Add the "ul" value for "what". (closes #7037)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 58c390e..6bf205d 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -10348,7 +10348,9 @@
 		"bg"		background color (as with "fg")
 		"font"		font name (only available in the GUI)
 				|highlight-font|
-		"sp"		special color (as with "fg") |highlight-guisp|
+		"sp"		special color for the GUI (as with "fg")
+				|highlight-guisp|
+		"ul"		underline color for cterm: number as a string
 		"fg#"		like "fg", but for the GUI and the GUI is
 				running the name in "#RRGGBB" form
 		"bg#"		like "fg#" for "bg"