Nudge people towards better hashes in a few places (#538)

diff --git a/doc/api/crypto.rst b/doc/api/crypto.rst
index 6276bc2..cac9dd9 100644
--- a/doc/api/crypto.rst
+++ b/doc/api/crypto.rst
@@ -127,7 +127,7 @@
 
     *key* is a :py:class:`PKey` instance.  *data* is a ``str`` instance.
     *digest* is a ``str`` naming a supported message digest type, for example
-    :py:const:`sha1`.
+    :py:const:`b"sha256"`.
 
     .. versionadded:: 0.11
 
@@ -140,7 +140,7 @@
     key which generated the signature.  *signature* is a *str* instance giving
     the signature itself.  *data* is a *str* instance giving the data to which
     the signature applies.  *digest* is a *str* instance naming the message
-    digest type of the signature, for example :py:const:`sha1`.
+    digest type of the signature, for example :py:const:`b"sha256"`.
 
     .. versionadded:: 0.11
 
diff --git a/src/OpenSSL/crypto.py b/src/OpenSSL/crypto.py
index 116cc51..84a1a07 100644
--- a/src/OpenSSL/crypto.py
+++ b/src/OpenSSL/crypto.py
@@ -937,7 +937,7 @@
         :param pkey: The key pair to sign with.
         :type pkey: :py:class:`PKey`
         :param digest: The name of the message digest to use for the signature,
-            e.g. :py:data:`b"sha1"`.
+            e.g. :py:data:`b"sha256"`.
         :type digest: :py:class:`bytes`
         :return: ``None``
         """
@@ -2071,7 +2071,7 @@
             :data:`FILETYPE_ASN1`, or :data:`FILETYPE_TEXT`.
         :param int days: The number of days until the next update of this CRL.
         :param bytes digest: The name of the message digest to use (eg
-            ``b"sha1"``).
+            ``b"sha2566"``).
         :rtype: bytes
         """