Add additional description to overview

Author: nikic

llvm/docs/LangRef.rst

@@ -11210,6 +11210,10 @@ Overview:
 
 The '``zext``' instruction zero extends its operand to type ``ty2``.
 
+The ``nneg`` (non-negative) flag, if present, specifies that the operand is
+non-negative. This property may be used by optimization passes to later
+convert the ``zext`` into a ``sext``.
+
 Arguments:
 """"""""""