doc: ngx.var: documented the values for undefined and uninitialized n…

…ginx variables. thanks Sean Johnson for asking for this explanation in #595.

README.markdown

@@ -2855,7 +2855,7 @@ many others are not, like `$query_string`, `$arg_PARAMETER`, and `$http_NAME`.
 Nginx regex group capturing variables `$1`, `$2`, `$3`, and etc, can be read by this
 interface as well, by writing `ngx.var[1]`, `ngx.var[2]`, `ngx.var[3]`, and etc.
 
-Setting `ngx.var.Foo` to a `nil` value will unset the `$Foo` Nginx variable. 
+Setting `ngx.var.Foo` to a `nil` value will unset the `$Foo` Nginx variable.
 
 ```lua
 
@@ -2872,6 +2872,8 @@ Setting `ngx.var.Foo` to a `nil` value will unset the `$Foo` Nginx variable.
 
 to prevent (temporary) memory leaking within the current request's lifetime. Another way of caching the result is to use the [ngx.ctx](#ngxctx) table.
 
+Undefined NGINX variables are evaluated to `nil` while uninitialized (but defined) NGINX variables are evaluated to an empty Lua string.
+
 This API requires a relatively expensive metamethod call and it is recommended to avoid using it on hot code paths.
 
 [Back to TOC](#nginx-api-for-lua)

doc/HttpLuaModule.wiki

@@ -2285,7 +2285,7 @@ many others are not, like <code>$query_string</code>, <code>$arg_PARAMETER</code
 Nginx regex group capturing variables <code>$1</code>, <code>$2</code>, <code>$3</code>, and etc, can be read by this
 interface as well, by writing <code>ngx.var[1]</code>, <code>ngx.var[2]</code>, <code>ngx.var[3]</code>, and etc.
 
-Setting <code>ngx.var.Foo</code> to a <code>nil</code> value will unset the <code>$Foo</code> Nginx variable. 
+Setting <code>ngx.var.Foo</code> to a <code>nil</code> value will unset the <code>$Foo</code> Nginx variable.
 
 <geshi lang="lua">
     ngx.var.args = nil
@@ -2300,6 +2300,8 @@ Setting <code>ngx.var.Foo</code> to a <code>nil</code> value will unset the <cod
 
 to prevent (temporary) memory leaking within the current request's lifetime. Another way of caching the result is to use the [[#ngx.ctx|ngx.ctx]] table.
 
+Undefined NGINX variables are evaluated to `nil` while uninitialized (but defined) NGINX variables are evaluated to an empty Lua string.
+
 This API requires a relatively expensive metamethod call and it is recommended to avoid using it on hot code paths.
 
 == Core constants ==