doc: documented that ngx.exit() is enabled in the context of ngx.timer.* callbacks.

README.markdown

@@ -4395,7 +4395,7 @@ ngx.exit
 --------
 **syntax:** *ngx.exit(status)*
 
-**context:** *rewrite_by_lua*, access_by_lua*, content_by_lua*, header_filter_by_lua**
+**context:** *rewrite_by_lua*, access_by_lua*, content_by_lua*, header_filter_by_lua*, ngx.timer.**
 
 When `status >= 200` (i.e., `ngx.HTTP_OK` and above), it will interrupt the execution of the current request and return status code to nginx.
 

doc/HttpLuaModule.wiki

@@ -3643,7 +3643,7 @@ Since <code>v0.8.3</code> this function returns <code>1</code> on success, or re
 == ngx.exit ==
 '''syntax:''' ''ngx.exit(status)''
 
-'''context:''' ''rewrite_by_lua*, access_by_lua*, content_by_lua*, header_filter_by_lua*''
+'''context:''' ''rewrite_by_lua*, access_by_lua*, content_by_lua*, header_filter_by_lua*, ngx.timer.*''
 
 When <code>status >= 200</code> (i.e., <code>ngx.HTTP_OK</code> and above), it will interrupt the execution of the current request and return status code to nginx.