Tweak RKP HAL comments

Change the cert_type field from an enum of strings to a tstr type with
the known types documented in comments. The types are part of the
protocol between the HAL implementation and the provisioning server that
is opaque to the Android platform, so there's not need to bump the HAL
version in order to add new certificate types.

Replace the undefined Dcc type/acronym with the term "DICE chain" for
smoother reading.

Make the behaviour of generateCertificateRequest() in the v3 HAL more
explicit by explaining that a ServiceSpecificException should be raised
with the same error code that is currently documented.

Bug: 240312857
Test: n/a - comments only
Change-Id: If5acc388b25fa24d240c936ddefd08943fc6dd8d