Document otel.instrumentation.external-annotations.include

[theletterf]

While documenting instrumentations' settings in #4981, @iNikem commented that we should better document the following property:

| System property 	| Type 	| Default 	| Description 	|
|-----------------	|------	|---------	|-------------	|
| `otel.instrumentation.external-annotations.include` | String	| Default annotations | Configuration for trace annotations, in the form of a pattern that matches `'package.Annotation$Name;*'`.

Link to the relevant comment: #4981 (comment)

[trask]

Closing, can revisit later if anyone has renewed interest.