updates MatchError godoc comment to also accept a Gomega matcher

docs/index.md

@@ -796,7 +796,7 @@ succeeds if `ACTUAL` is a non-nil `error` that matches `EXPECTED`. `EXPECTED` mu
 - A matcher, in which case `ACTUAL.Error()` is tested against the matcher.
 - An error, in which case `ACTUAL` and `EXPECTED` are compared via `reflect.DeepEqual()`. If they are not deeply equal, they are tested by `errors.Is(ACTUAL, EXPECTED)`. (The latter allows to test whether `ACTUAL` wraps an `EXPECTED` error.)
 
-Any other type for `EXPECTED` is an error.
+Any other type for `EXPECTED` is an error. It is also an error for `ACTUAL` to be nil.
 
 ### Working with Channels
 

matchers.go

@@ -87,14 +87,17 @@ func Succeed() types.GomegaMatcher {
 	return &matchers.SucceedMatcher{}
 }
 
-// MatchError succeeds if actual is a non-nil error that matches the passed in string/error.
+// MatchError succeeds if actual is a non-nil error that matches the passed in
+// string, error, or matcher.
 //
 // These are valid use-cases:
 //
-//	Expect(err).Should(MatchError("an error")) //asserts that err.Error() == "an error"
-//	Expect(err).Should(MatchError(SomeError)) //asserts that err == SomeError (via reflect.DeepEqual)
+//  Expect(err).Should(MatchError("an error")) //asserts that err.Error() == "an error"
+//  Expect(err).Should(MatchError(SomeError)) //asserts that err == SomeError (via reflect.DeepEqual)
+//  Expect(err).Should(MatchError(ContainsSubstring("sprocket not found"))) // asserts that edrr.Error() contains substring "sprocket not found"
 //
-// It is an error for err to be nil or an object that does not implement the Error interface
+// It is an error for err to be nil or an object that does not implement the
+// Error interface
 func MatchError(expected interface{}) types.GomegaMatcher {
 	return &matchers.MatchErrorMatcher{
 		Expected: expected,