Document Java time chrono era values

[jonathanpeppers]

Refs #257

Documents all seven Android-bound enum constants in Java.Time.Chrono.IsoEra, MinguoEra, ThaiBuddhistEra, and HijrahEra with their names and numeric values.

Sources

• https://developer.android.com/reference/java/time/chrono/IsoEra
• https://developer.android.com/reference/java/time/chrono/MinguoEra
• https://developer.android.com/reference/java/time/chrono/ThaiBuddhistEra
• https://developer.android.com/reference/java/time/chrono/HijrahEra

Validation

• Parsed all four changed XML files.
• Confirmed CRLF line endings in all changed files.
• Ran git diff --check.
• Scoped Java.Time.Chrono placeholder count is 349 (seven documented members; remaining namespace gaps are intentionally out of scope).

[Copilot]

Pull request overview

This PR fills in XML documentation placeholders in the Java.Time.Chrono namespace by documenting the Android-exposed era singleton properties for IsoEra, MinguoEra, ThaiBuddhistEra, and HijrahEra, including their era names and numeric getValue() mappings.

Changes:

• Documented IsoEra.BCE (0) and IsoEra.CE (1) with summaries, values, remarks, and reference links.
• Documented MinguoEra.BEFORE_ROC (0) and MinguoEra.ROC (1) with summaries, values, remarks, and reference links.
• Documented ThaiBuddhistEra.BEFORE_BE (0) and ThaiBuddhistEra.BE (1), plus HijrahEra.AH (1), with summaries, values, remarks, and reference links (including AOSP attribution text).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
docs/xml/Java.Time.Chrono/IsoEra.xml	Replaces placeholders with era constant docs for BCE/CE, including numeric values and Android reference links.
docs/xml/Java.Time.Chrono/MinguoEra.xml	Replaces placeholders with docs for BEFORE_ROC/ROC, including numeric values and Android reference links.
docs/xml/Java.Time.Chrono/ThaiBuddhistEra.xml	Replaces placeholders with docs for BEFORE_BE/BE, including numeric values and Android reference links.
docs/xml/Java.Time.Chrono/HijrahEra.xml	Replaces placeholders with docs for AH, including numeric value and Android reference link.

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[jonathanpeppers]

@dalexsoto review

[dalexsoto]

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