doc: ngx.var: named more readonly nginx builtin variables, as per adamonduty's request in openresty#440.

Author: agentzh

README.markdown

@@ -2550,7 +2550,7 @@ For example:
 That is, nginx variables cannot be created on-the-fly.
 
 Some special nginx variables like `$args` and `$limit_rate` can be assigned a value,
-some are not, like `$arg_PARAMETER`.
+many others are not, like `$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.

doc/HttpLuaModule.wiki

@@ -2027,7 +2027,7 @@ For example:
 That is, nginx variables cannot be created on-the-fly.
 
 Some special nginx variables like <code>$args</code> and <code>$limit_rate</code> can be assigned a value,
-some are not, like <code>$arg_PARAMETER</code>.
+many others are not, like <code>$query_string</code>, <code>$arg_PARAMETER</code>, and <code>$http_NAME</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.