Add System.Speech missing docs

[carlossanlop]

Contributes to dotnet/runtime#87711

Finish documenting the APIs that were still incomplete.

[learn-build-service-prod]

Learn Build status updates of commit b3876f9:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Speech.Recognition.SrgsGrammar/SrgsRulesCollection.xml	✅Succeeded	View
xml/System.Speech.Synthesis.TtsEngine/ITtsEngineSite.xml	✅Succeeded	View
xml/System.Speech.Synthesis.TtsEngine/Prosody.xml	✅Succeeded	View
xml/System.Speech.Synthesis/SpeechSynthesizer.xml	✅Succeeded	View

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