patch 8.2.4770: cannot easily mix expression and heredoc

Problem:    Cannot easily mix expression and heredoc.
Solution:   Support  in heredoc. (Yegappan Lakshmanan, closes #10138)

runtime/doc/eval.txt

@@ -3224,14 +3224,32 @@ declarations and assignments do not use a command.  |vim9-declaration|
 
 						*:let=<<* *:let-heredoc*
 					*E990* *E991* *E172* *E221* *E1145*
-:let {var-name} =<< [trim] {endmarker}
+:let {var-name} =<< [trim] [eval] {endmarker}
 text...
 text...
 {endmarker}
 			Set internal variable {var-name} to a |List|
 			containing the lines of text bounded by the string
-			{endmarker}. The lines of text is used as a
-			|literal-string|.
+			{endmarker}.
+
+			If "eval" is not specified, then each line of text is
+			used as a |literal-string|.  If "eval" is specified,
+			then any Vim expression in the form ``={expr}`` is
+			evaluated and the result replaces the expression.
+			Example where $HOME is expanded: >
+				let lines =<< trim eval END
+				  some text
+				  See the file `=$HOME`/.vimrc
+				  more text
+				END
+<			There can be multiple Vim expressions in a single line
+			but an expression cannot span multiple lines.  If any
+			expression evaluation fails, then the assignment fails.
+			once the "`=" has been found {expr} and a backtick
+			must follow.  {expr} cannot be empty.
+			Currenty, in a compiled function {expr} is evaluated
+			when compiling the function, THIS WILL CHANGE.
+
 			{endmarker} must not contain white space.
 			{endmarker} cannot start with a lower case character.
 			The last line should end only with the {endmarker}
@@ -3281,6 +3299,13 @@ text...
 					1 2 3 4
 					5 6 7 8
 				DATA
+
+				let code =<< trim eval CODE
+				   let v = `=10 + 20`
+				   let h = "`=$HOME`"
+				   let s = "`=Str1()` abc `=Str2()`"
+				   let n = `=MyFunc(3, 4)`
+				CODE
 <
 								*E121*
 :let {var-name}	..	List the value of variable {var-name}.  Multiple