Rename mbedtls_pk_signature_size to mbedtls_pk_get_signature_size
Add more descriptive comments and return value description
diff --git a/include/mbedtls/pk_info.h b/include/mbedtls/pk_info.h
index d1d95ef..a8b735f 100644
--- a/include/mbedtls/pk_info.h
+++ b/include/mbedtls/pk_info.h
@@ -120,9 +120,11 @@
* usage restrictions into account. */
int (*can_do)( const void * ctx, mbedtls_pk_type_t type );
- /** Signature size
+ /** Upper bound of the signature length
*
- * mbedtls_pk_signature_size() returns this value.
+ * mbedtls_pk_get_signature_size() returns this value.
+ *
+ * In case of an error, or an unsupported key type, 0 should be returned.
*
* Opaque implementations may omit this method if they do not support
* signature. */