Document the erroneous removal of pk_ops.h

doc/migration_guide.rst

@@ -26,6 +26,14 @@ algorithm headers (such as ``aes.h``) have been removed. Instead you should
 create objects via the factory methods (in the case of AES,
 ``BlockCipher::create``) which works in both 2.x and 3.0
 
+Errata: ``pk_ops.h``
+^^^^^^^^^^^^^^^^^^^^
+
+Between Botan 3.0 and 3.2 the public header ``pk_ops.h`` was removed
+accidentally. This header is typically required for specialized applications
+that interface with dedicated crypto hardware. If you are migrating such an
+application, please make sure to use Botan 3.3 or newer.
+
 Build Artifacts
 ---------------
 

src/lib/pubkey/pk_ops.h

@@ -16,6 +16,11 @@
 * Unless you're doing something like that, you don't need anything
 * here. Instead use pubkey.h which wraps these types safely and
 * provides a stable application-oriented API.
+*
+* Note: This header was accidentally pulled from the public API between
+*       Botan 3.0.0 and 3.2.0, and then restored in 3.3.0. If you are
+*       maintaining an application which used this header in Botan 2.x,
+*       you should make sure to use Botan 3.3.0 or later when migrating.
 */
 
 #include <botan/pk_keys.h>