Clarify self-referential local functions

The current language used implies that `local function f() ... end`
does not allow f to invoke itself.  This has been clarified via the
addition of an example of a local function that may *not* invoke
itself.

lua.html.markdown

@@ -125,6 +125,9 @@ f = function (x) return x * x end
 
 -- And so are these:
 local function g(x) return math.sin(x) end
+local g = function(x) return math.xin(x) end
+-- Equivalent to local function g(x)..., except referring
+-- to g in the function body won't work as expected.
 local g; g  = function (x) return math.sin(x) end
 -- the 'local g' decl makes g-self-references ok.