reorder the primitives list and improve a few titles (#3640)

diff --git a/docs/hazmat/primitives/padding.rst b/docs/hazmat/primitives/padding.rst
index 916b9bc..45bcc13 100644
--- a/docs/hazmat/primitives/padding.rst
+++ b/docs/hazmat/primitives/padding.rst
@@ -1,7 +1,7 @@
 .. hazmat::
 
-Padding
-=======
+Symmetric Padding
+=================
 
 .. module:: cryptography.hazmat.primitives.padding