patch 8.0.1510: cannot test if a command causes a beep

Problem:    Cannot test if a command causes a beep.
Solution:   Add assert_beeps().
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 9158f38..8f14ddd 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2017,6 +2017,7 @@
 arglistid([{winnr} [, {tabnr}]]) Number	argument list id
 argv({nr})			String	{nr} entry of the argument list
 argv()				List	the argument list
+assert_beeps({cmd})		none	assert {cmd} causes a beep
 assert_equal({exp}, {act} [, {msg}])
 				none	assert {exp} is equal to {act}
 assert_exception({error} [, {msg}])
@@ -2568,6 +2569,11 @@
 <		Without the {nr} argument a |List| with the whole |arglist| is
 		returned.
 
+assert_beeps({cmd})					*assert_beeps()*
+		Run {cmd} and add an error message to |v:errors| if it does
+		NOT produce a beep or visual bell.
+		Also see |assert_fails()|.
+
 							*assert_equal()*
 assert_equal({expected}, {actual} [, {msg}])
 		When {expected} and {actual} are not equal an error message is
@@ -2600,6 +2606,8 @@
 		Run {cmd} and add an error message to |v:errors| if it does
 		NOT produce an error.
 		When {error} is given it must match in |v:errmsg|.
+		Note that beeping is not considered an error, and some failing
+		commands only beep.  Use |assert_beeps()| for those.
 
 assert_false({actual} [, {msg}])				*assert_false()*
 		When {actual} is not false an error message is added to