Document MethodHandle reference kinds

[jonathanpeppers]

Summary

• Documented the Java.Lang.Invoke.ReferenceKind enum and all nine reference-kind values.
• Kept the change limited to source-backed method-handle field, method, and constructor access behavior.

Refs #201

Sources

• https://developer.android.com/reference/java/lang/invoke/MethodHandleInfo
• https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/invoke/MethodHandleInfo.html

Validation

• XML parse of docs/xml/Java.Lang.Invoke/ReferenceKind.xml
• Confirmed CRLF line endings and no bare LF characters
• git diff --check
• Confirmed all 10 placeholders in ReferenceKind.xml were replaced

[Copilot]

Pull request overview

This PR replaces placeholder XML documentation for Java.Lang.Invoke.ReferenceKind with concrete summaries describing the MethodHandleInfo reference kinds used for field, method, and constructor access.

Changes:

• Documented the Java.Lang.Invoke.ReferenceKind enum summary.
• Replaced all nine enum value <summary> placeholders with descriptions matching their reference-kind behavior.

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[jonathanpeppers]

@dalexsoto review

[dalexsoto]

The documented API behavior matches the current contracts; no blocking issues found.