Document initialDirectory limitation for picking on Android

[vicajilau]

Summary

• Follow up from the review discussion on Forward initialDirectory and dialogTitle to native on Darwin #2218: Android has a similar looking gap to the Darwin one fixed there, but it goes deeper.
• The native Kotlin side never sets DocumentsContract.EXTRA_INITIAL_URI for ACTION_OPEN_DOCUMENT, ACTION_OPEN_DOCUMENT_TREE, or the ACTION_GET_CONTENT variants, so initialDirectory has no effect on pickFile(), pickFiles(), pickFileAndDirectoryPaths(), or getDirectoryPath(). Only saveFile() honors it today.
• Making it work for picking needs native support for turning a plain path into a SAF document tree URI, which isn't a direct mapping, so it's documented as a known limitation for now rather than fixed in this PR.

No code changes, documentation only.

[vicajilau]

Wrong target, moved this commit onto #2218 instead.