Update ACTION_BIOMETRIC_ENROLL documentation

Activity result codes are already being used "undocumentedly" by SUW.
Given the timeline, it's better to not make any guarantees about the
result code and just mention that applications should still check
BiometricManager#canAuthenticate(int) after onActivityResult.

Fixes: 151058692

Test: Skip fingerprint in SUW. Fingerprint SUW is not shown again.
Change-Id: Ia36ea1b6cfc39848be00f8d2219604e68ff45213