patch 8.1.1723: heredoc assignment has no room for new features

Problem:    Heredoc assignment has no room for new features. (FUJIWARA Takuya)
Solution:   Require the marker does not start with a lower case character.
            (closes #4705)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 1fa42b3..6a44c9f 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -11386,7 +11386,8 @@
 			Like above, but append/add/subtract the value for each
 			|List| item.
 
-					*:let=<<* *:let-heredoc* *E990* *E991*
+						*:let=<<* *:let-heredoc*
+						*E990* *E991* *E172* *E221*
 :let {var-name} =<< [trim] {marker}
 text...
 text...
@@ -11394,11 +11395,10 @@
 			Set internal variable {var-name} to a List containing
 			the lines of text bounded by the string {marker}.
 			{marker} must not contain white space.
+			{marker} cannot start with a lower case character.
 			The last line should end only with the {marker} string
 			without any other character.  Watch out for white
 			space after {marker}!
-			If {marker} is not supplied, then "." is used as the
-			default marker.
 
 			Without "trim" any white space characters in the lines
 			of text are preserved.  If "trim" is specified before