Put appropriate module attributes into doc fragments.

plugins/doc_fragments/module_certificate.py

@@ -16,6 +16,9 @@ class ModuleDocFragment(object):
 description:
   - This module allows one to (re)generate OpenSSL certificates.
   - It uses the cryptography python library to interact with OpenSSL.
+attributes:
+  diff_mode:
+    support: full
 requirements:
   - cryptography >= 1.6 (if using V(selfsigned) or V(ownca) provider)
 options:

plugins/doc_fragments/module_csr.py

@@ -15,6 +15,9 @@ class ModuleDocFragment(object):
 description:
   - This module allows one to (re)generate OpenSSL certificate signing requests.
   - This module supports the subjectAltName, keyUsage, extendedKeyUsage, basicConstraints and OCSP Must Staple extensions.
+attributes:
+  diff_mode:
+    support: full
 requirements:
   - cryptography >= 1.3
 options:

plugins/doc_fragments/module_privatekey.py

@@ -17,6 +17,9 @@ description:
     L(ECC,https://en.wikipedia.org/wiki/Elliptic-curve_cryptography) or L(EdDSA,https://en.wikipedia.org/wiki/EdDSA) private
     keys.
   - Keys are generated in PEM format.
+attributes:
+  diff_mode:
+    support: full
 requirements:
   - cryptography >= 1.2.3 (older versions might work as well)
 options:

plugins/doc_fragments/module_privatekey_convert.py

@@ -14,6 +14,9 @@ class ModuleDocFragment(object):
     DOCUMENTATION = r"""
 requirements:
   - cryptography >= 1.2.3 (older versions might work as well)
+attributes:
+  diff_mode:
+    support: none
 options:
   src_path:
     description: