doc: setting ngx.header.HEADER no longer throws out an exception when the header is already sent out; it now just logs an error message.

Signed-off-by: Yichun Zhang (agentzh) <agentzh@gmail.com>

Author: yandongxiao

README.markdown

@@ -4121,7 +4121,7 @@ The same applies to assigning an empty table:
  ngx.header["X-My-Header"] = {};
 ```
 
-Setting `ngx.header.HEADER` after sending out response headers (either explicitly with [ngx.send_headers](#ngxsend_headers) or implicitly with [ngx.print](#ngxprint) and similar) will throw out a Lua exception.
+Setting `ngx.header.HEADER` after sending out response headers (either explicitly with [ngx.send_headers](#ngxsend_headers) or implicitly with [ngx.print](#ngxprint) and similar) will log an error message.
 
 Reading `ngx.header.HEADER` will return the value of the response header named `HEADER`.
 

doc/HttpLuaModule.wiki

@@ -3406,7 +3406,7 @@ The same applies to assigning an empty table:
     ngx.header["X-My-Header"] = {};
 </geshi>
 
-Setting <code>ngx.header.HEADER</code> after sending out response headers (either explicitly with [[#ngx.send_headers|ngx.send_headers]] or implicitly with [[#ngx.print|ngx.print]] and similar) will throw out a Lua exception.
+Setting <code>ngx.header.HEADER</code> after sending out response headers (either explicitly with [[#ngx.send_headers|ngx.send_headers]] or implicitly with [[#ngx.print|ngx.print]] and similar) will log an error message.
 
 Reading <code>ngx.header.HEADER</code> will return the value of the response header named <code>HEADER</code>.