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