[Serializer] '*' context group serializes all properties including those without groups

[Maxcastel]

While reading the documentation, I thought that '*' should only serialize properties that have at least one group, so I opened an issue on symfony/symfony (#62912)

Since the issue was closed, I’m clarifying the documentation based on the explanation provided there symfony/symfony#62912 (comment):

Additionally, the example in the documentation may be misleading. In the shown result, all properties are serialized, but in the corresponding class definition, all properties have at least one group. This makes it impossible to determine whether all properties are serialized because '*' selects all properties, or because '*' selects only properties with at least one group.

to make this behavior explicit and avoid confusion for other readers.