doc: documented the new ngx.worker.pid() API.
refactor: removed an unused parameter, "len", in ngx_http_lua_ffi_script_eval_data.
doc: fixed linkrot of mod_lua. thanks Tatsuhiko Kubo for the patch in #336.
doc: fixed a typo in the doc for coroutine.yield(). thanks Ruoshan Huang for the report.