patch 9.1.1070: Cannot control cursor positioning of getchar()

Problem:  Cannot control cursor positioning of getchar().
Solution: Add "cursor" flag to {opts}, with possible values "hide",
          "keep" and "msg".

related: #10603
closes: #16569

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 46c3ba8..39ff14e 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -3953,6 +3953,13 @@
 		The optional argument {opts} is a Dict and supports the
 		following items:
 
+			cursor		A String specifying cursor behavior
+					when waiting for a character.
+					"hide": hide the cursor.
+					"keep": keep current cursor unchanged.
+					"msg": move cursor to message area.
+					(default: "msg")
+
 			number		If |TRUE|, return a Number when getting
 					a single character.
 					If |FALSE|, the return value is always