patch 8.2.3849: functions implementing reduce and map are too long

Problem:    Functions implementing reduce and map are too long.
Solution:   Use a function for each type of value.  Add a few more test cases
            and add to the help. (Yegappan Lakshmanan, closes #9370)

runtime/doc/eval.txt

@@ -4893,7 +4893,8 @@ filter({expr1}, {expr2})				*filter()*
 		of the current item.  For a |Dictionary| |v:key| has the key
 		of the current item and for a |List| |v:key| has the index of
 		the current item.  For a |Blob| |v:key| has the index of the
-		current byte.
+		current byte. For a |String| |v:key| has the index of the
+		current character.
 		Examples: >
 			call filter(mylist, 'v:val !~ "OLD"')
 <		Removes the items where "OLD" appears. >
@@ -7588,7 +7589,8 @@ map({expr1}, {expr2})					*map()*
 		of the current item.  For a |Dictionary| |v:key| has the key
 		of the current item and for a |List| |v:key| has the index of
 		the current item.  For a |Blob| |v:key| has the index of the
-		current byte.
+		current byte. For a |String| |v:key| has the index of the
+		current character.
 		Example: >
 			:call map(mylist, '"> " . v:val . " <"')
 <		This puts "> " before and " <" after each item in "mylist".
@@ -8959,9 +8961,9 @@ readfile({fname} [, {type} [, {max}]])
 
 reduce({object}, {func} [, {initial}])			*reduce()* *E998*
 		{func} is called for every item in {object}, which can be a
-		|String|, |List| or a |Blob|.  {func} is called with two arguments:
-		the result so far and current item.  After processing all
-		items the result is returned.
+		|String|, |List| or a |Blob|.  {func} is called with two
+		arguments: the result so far and current item.  After
+		processing all items the result is returned.
 
 		{initial} is the initial result.  When omitted, the first item
 		in {object} is used and {func} is first called for the second