imptcp: no longer limited to 16 threads as stated in the documentation

[maxime-leroy]

Since commit 873c7dd59156 ("make imptcp worker-pool size configurable (without max bound)"), the imptcp module is no longer limited to a maximum of 16 threads. However, the documentation still states:
"No more than 16 threads can be set (if tried to, rsyslog always resorts to 16)."

Please update the documentation to reflect the removal of this limitation.