Documentation for semantic annotation extension to Scala IDE

Documentation for semantic annotation extension to Scala IDE (task #1001989, https://github.com/scala-ide/scala-ide/pull/644)

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It would be good to have the required imports:

```scala
import org.scalaide.ui.internal.editor.decorators.custom.AllMethodsTraverserDef
import org.scalaide.ui.internal.editor.decorators.custom.TraverserDef.TypeDefinition
import org.scalaide.ui.internal.editor.decorators.SemanticAction
import org.scalaide.ui.internal.editor.decorators.custom.CustomSemanticAction
import org.scalaide.ui.internal.editor.decorators.semantichighlighting.SemanticHighlightingReconciliationParticipant
import org.scalaide.ui.internal.editor.decorators.semantichighlighting.SemanticHighlightingReconciliation
import org.eclipse.jdt.internal.ui.javaeditor.JavaSourceViewer
```