cc: Add docstring for IsVndk() and UseVndk()

Note that IsVndk() returns true for all variants of vndk-enabled
libraries. Not only vendor variant, but also platform and product
variants of vndk-enabled library return true.

OTOH, `UseVndk() && IsVndk()` returns true for vendor and product
variants of vndk-enabled libraries, which actually include the actual
VNDK libs (which goes into the VNDK apex) and VNDK extension libs (which
goes to /product or /vendor partition).

Bug: 259361996
Test: Presubmit
Change-Id: Ib735fe0f7433284ca768d0e251a4f599fff5185c