Add missing low-level API documentation

[DirectXMan12]

The following "public" low-level API files are missing documentation:

• chan_bindings.pyx
• oids.pyx
• ext_rfc5588.pyx
• ext_cred_store.pyx
• exceptions.pyx (probably just document file itself, not error classes)
• creds.pyx (inquire_cred, inquire_cred_by_mech)

The following "private" low-level API files are missing documentation:

• types.pyx (GenericFlagSet and IntEnumFlagSet)
• mech_krb5.pyx
• cython_converters.pyx (c_make_oid)