Clarify liberty of implementations to provide separate client and server jars

Although the API jar cannot be split, most implementations do provide separate jars to avoid always referencing a large JAR containing all classes. For example, a JAX-RS client only application does not need server classes from the implementation.

It was suggested that this should be explicitly allowed by the spec: Here is some text that we can use for that,

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> An implementation of JAX-RS must provide all capabilities required by this specification, but may package the capabilities so that an application need only include either the client capabilities of the server capabilities. If the client capabilities are not included but are used, the following APIs will fail in the following manner. If the server capabilities are not included but are used, the following APIs will fail in the following manner.

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