patch 8.2.0893: assert_equalfile() does not take a third argument

Problem:    Assert_equalfile() does not take a third argument.
Solution:   Implement the third argument. (Gary Johnson)
diff --git a/runtime/doc/testing.txt b/runtime/doc/testing.txt
index 9c87cba..19fb321 100644
--- a/runtime/doc/testing.txt
+++ b/runtime/doc/testing.txt
@@ -263,9 +263,8 @@
 		Can also be used as a |method|: >
 			mylist->assert_equal([1, 2, 3])
 
-
 <							*assert_equalfile()*
-assert_equalfile({fname-one}, {fname-two})
+assert_equalfile({fname-one}, {fname-two} [, {msg}])
 		When the files {fname-one} and {fname-two} do not contain
 		exactly the same text an error message is added to |v:errors|.
 		Also see |assert-return|.
@@ -276,7 +275,6 @@
 		Can also be used as a |method|: >
 			GetLog()->assert_equalfile('expected.log')
 
-
 assert_exception({error} [, {msg}])			*assert_exception()*
 		When v:exception does not contain the string {error} an error
 		message is added to |v:errors|.  Also see |assert-return|.