Bluetooth: Document bt_conn_le_cs_mode and clarify encoding notes

Add API documentation for `bt_conn_le_cs_mode` and internal extractors
for main and sub-mode parts. Clarify that the enum values are an
internal encoding, not a stable API, and that the sub-mode extractor
maps 0 to HCI 0xFF (unused) for a compact representation.

Also mark the extractors as private helpers and tighten wording for
developer clarity.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>