csrgen: drive-by docstring

Part of: https://pagure.io/freeipa/issue/7496

Reviewed-By: Christian Heimes <cheimes@redhat.com>

ipaclient/csrgen_ffi.py

@@ -220,6 +220,12 @@ def _parse_dn_section(subj, dn_sk):
 
 
 def build_requestinfo(config, public_key_info):
+    '''
+    Return a cffi buffer containing a DER-encoded CertificationRequestInfo.
+
+    The returned object implements the buffer protocol.
+
+    '''
     reqdata = NULL
     req = NULL
     nconf_bio = NULL