add more prose about how the digest is generated
diff --git a/docs/x509/reference.rst b/docs/x509/reference.rst
index 61f73e9..e83a4ac 100644
--- a/docs/x509/reference.rst
+++ b/docs/x509/reference.rst
@@ -1125,7 +1125,9 @@
Creates a new SubjectKeyIdentifier instance using the public key
provided to generate the appropriate digest. This should be the public
- key that is in the certificate.
+ key that is in the certificate. The generated digest is the SHA1 hash
+ of the ``subjectPublicKey`` ASN.1 bit string. This is the first
+ recommendation in :rfc:`5280` section 4.2.1.2.
:param public_key: One of
:class:`~cryptography.hazmat.primitives.asymmetric.rsa.RSAPublicKey`