Improve documentation for psa_crypto_cipher.h

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/psa_crypto_cipher.h b/library/psa_crypto_cipher.h
index fae9847..3bd5360 100644
--- a/library/psa_crypto_cipher.h
+++ b/library/psa_crypto_cipher.h
@@ -1,5 +1,5 @@
 /*
- *  PSA cipher driver entry points
+ *  PSA cipher driver entry points and associated auxiliary functions
  */
 /*
  *  Copyright The Mbed TLS Contributors