add some missing docstrings
diff --git a/OpenSSL/SSL.py b/OpenSSL/SSL.py
index 1e85795..b0b6395 100644
--- a/OpenSSL/SSL.py
+++ b/OpenSSL/SSL.py
@@ -105,7 +105,9 @@
 
 
 class Error(Exception):
-    pass
+    """
+    An error occurred in an `OpenSSL.SSL` API.
+    """
 
 
 
diff --git a/OpenSSL/crypto.py b/OpenSSL/crypto.py
index e699f37..f47bb14 100644
--- a/OpenSSL/crypto.py
+++ b/OpenSSL/crypto.py
@@ -18,7 +18,9 @@
 
 
 class Error(Exception):
-    pass
+    """
+    An error occurred in an `OpenSSL.crypto` API.
+    """
 
 
 _raise_current_error = partial(_exception_from_error_queue, Error)
diff --git a/OpenSSL/rand.py b/OpenSSL/rand.py
index 0204118..de158f6 100644
--- a/OpenSSL/rand.py
+++ b/OpenSSL/rand.py
@@ -15,7 +15,9 @@
 
 
 class Error(Exception):
-    pass
+    """
+    An error occurred in an `OpenSSL.rand` API.
+    """
 
 _raise_current_error = partial(_exception_from_error_queue, Error)