Googletest export

Add a breadcrumb about nullopt comparison near the Optional() matcher.

Also add a note about how otherwise-spurious 'Eq()' may be needed in some cases. Without this, something like Field(&MyStruct::optional_field_without_equals_equals, absl::nullopt) doesn't work - it converts the nullopt to an optional<> of the non-equalable type, and fails to select the operator==(optional<>, nullopt_t) overload. The Eq() lets the type persist later into the match.

PiperOrigin-RevId: 281305519

googlemock/docs/cheat_sheet.md

@@ -266,7 +266,7 @@ Matcher                     | Description
 | `IsTrue()`             | `argument` evaluates to `true` in a Boolean context. |
 | `IsNull()`             | `argument` is a `NULL` pointer (raw or smart).      |
 | `NotNull()`            | `argument` is a non-null pointer (raw or smart).    |
-| `Optional(m)`          | `argument` is `optional<>` that contains a value matching `m`. |
+| `Optional(m)`          | `argument` is `optional<>` that contains a value matching `m`. (For testing whether an `optional<>` is set, check for equality with `nullopt`. You may need to use `Eq(nullopt)` if the inner type doesn't have `==`.)|
 | `VariantWith<T>(m)`    | `argument` is `variant<>` that holds the alternative of type T with a value matching `m`. |
 | `Ref(variable)`        | `argument` is a reference to `variable`.            |
 | `TypedEq<type>(value)` | `argument` has type `type` and is equal to `value`. You may need to use this instead of `Eq(value)` when the mock function is overloaded. |