Update documentation for KEY_CARRIER_NAME_STRING

The documentation for KEY_CARRIER_NAME_STRING was out
of date; documentation updated to support the additional
use case of "fallback" carrier name when SPN is
unavailable.

Bug: 71882065
Test: compilation (docstring-only change)
Change-Id: I8e6a44358f8b18384c3c6e3cc0964e94c2484a11
1 file changed