Added missing modules to lib.txt and warnings to some module's docs.

lib/pure/poly.nim

@@ -7,6 +7,9 @@
 #    distribution, for details about the copyright.
 #
 
+## **Warning:** This module will be moved out of the stdlib and into a
+## Nimble package, don't use it.
+
 import math
 import strutils
 import numeric