patch 8.1.1354: getting a list of text lines is clumsy

Problem:    Getting a list of text lines is clumsy.
Solution:   Add the =<< assignment. (Yegappan Lakshmanan, closes #4386)

runtime/doc/eval.txt

@@ -11416,6 +11416,44 @@ This does NOT work: >
 			Like above, but append/add/subtract the value for each
 			|List| item.
 
+					*:let=<<* *:let-heredoc* *E990* *E991*
+:let {var-name} =<< [trim] {marker}
+text...
+text...
+{marker}
+			Set internal variable {var-name} to a List containing
+			the lines of text bounded by the string {marker}.
+			{marker} must not contain white space.
+			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.
+
+			Any white space characters in the lines of text are
+			preserved.  If "trim" is specified before {marker},
+			then all the leading indentation exactly matching the
+			leading indentation before `let` is stripped from the
+			input lines and the line containing {marker}.  Note
+			that the difference between space and tab matters
+			here.
+
+			If {var-name} didn't exist yet, it is created.
+			Cannot be followed by another command, but can be
+			followed by a comment.
+
+			Examples: >
+				let var1 =<< END
+			Sample text 1
+			    Sample text 2
+			Sample text 3
+			END
+
+				let data =<< trim DATA
+				1 2 3 4
+				5 6 7 8
+				DATA
+<
 								*E121*
 :let {var-name}	..	List the value of variable {var-name}.  Multiple
 			variable names may be given.  Special names recognized