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