Document the hardlimit of HiddenServiceAuthorizeClient basic

I ran some tests on HiddenServiceAuthorizeClient basic auth-type and found that it stopped working when I created 49 or more clients. I started with 10 clients and kept adding 10 more at a time. When I had 39 clients, the hidden service worked, but when I added 10 more, the hostname and client_keys were generated as expected, but hidden service stopped working for all of the clients.

HiddenServiceDir /var/lib/tor/test_public/ # tlxnxx74fpmkw2qh.onion HiddenServicePort 80 127.0.0.1:80 HiddenServiceAuthorizeClient basic
tlx_cl01,
tlx_cl02,
tlx_cl03,
... tlx_cl47,
tlx_cl48,
tlx_cl49

According to the man page and the specs, the stealth mode doesn't work for more than 16 clients, but implied that the basic mode should work.