doc(config): add a section for worker and thread numbers requirements

Author: sergehardy

docs/worker.md

@@ -127,6 +127,7 @@ docker run \
     dunglas/frankenphp
 ```
 
+
 ### Restart the Worker After a Certain Number of Requests
 
 As PHP was not originally designed for long-running processes, there are still many libraries and legacy codes that leak memory.
@@ -163,6 +164,13 @@ frankenphp {
 }
 ```
 
+### Number of threads
+
+When the worker mode is enabled, an additional thread is required to handle the incoming requests,
+so the default `num_threads` value is `1 + 2x(number of available CPUs)`.
+
+Thus, in that mode if `num_threads` and `num` are explicitly set, `num_threads` must be strictly greater then `num`.
+
 ## Superglobals Behavior
 
 [PHP superglobals](https://www.php.net/manual/en/language.variables.superglobals.php) (`$_SERVER`, `$_ENV`, `$_GET`...)