Add Haddock documentation headers

Very minimal -- basically copying the header sections from HaskellNet so
that it's easy to see where the function would belong were it part of
HaskellNet itself.

src/Network/HaskellNet/IMAP/SSL.hs

@@ -1,5 +1,6 @@
 module Network.HaskellNet.IMAP.SSL
-    ( connectIMAPSSL
+    ( -- * Establishing connection
+      connectIMAPSSL
     , connectIMAPSSLPort
     ) where
 

src/Network/HaskellNet/POP3/SSL.hs

@@ -1,5 +1,6 @@
 module Network.HaskellNet.POP3.SSL
-    ( connectPop3SSL
+    ( -- * Establishing connection
+      connectPop3SSL
     , connectPop3SSLPort
     ) where
 

src/Network/HaskellNet/SMTP/SSL.hs

@@ -1,8 +1,10 @@
 module Network.HaskellNet.SMTP.SSL
-    ( connectSMTPSSL
+    ( -- * Establishing connection
+      connectSMTPSSL
     , connectSMTPSSLPort
     , connectSMTPSTARTTLS
     , connectSMTPSTARTTLSPort
+      -- * Other Useful Operations
     , doSMTPSSL
     , doSMTPSSLPort
     , doSMTPSTARTTLS