NSEdoc cleanup

Mostly splitting function summaries (the first paragraph of NSEdoc) from
the body of the description to make the summary indexes shorter and
easier to scan.

Also fixed some unbalanced code tags like <code>foo</table>

nselib/creds.lua

@@ -277,7 +277,7 @@ Credentials = {
   --- Returns a credential iterator
   --
   -- @see State
-  -- @param state mask containing values from the <Code>State</code> table
+  -- @param state mask containing values from the <code>State</code> table
   -- @return credential iterator, returning a credential each time it's
   --         called. Unless filtered by the state mask all credentials
   --         for the host, port match are iterated over.