[DOC] extra_port feature confusion in documentation

[Przemek]

In https://www.percona.com/doc/percona-server/5.7/performance/threadpool.html#extra_port
we can read:
"This can be used in case no new connections can be established due to all worker threads being busy or being locked when pool-of-threads feature is enabled."

It is also described only in Thread Pool documentation page.

This seems to suggest that this feature is only working and depending on the thread_handling=pool-of-threads, while this is not true, as it works in default mode too:

mysql [127.0.0.1] {msandbox} ((none)) > select @@thread_handling,@@extra_port,@@extra_max_connections;
+---------------------------+--------------+-------------------------+
| @@thread_handling | @@extra_port | @@extra_max_connections |
+---------------------------+--------------+-------------------------+
| one-thread-per-connection | 2333 | 10 |
+---------------------------+--------------+-------------------------+
1 row in set (0.00 sec)

mysql [127.0.0.1] {msandbox} ((none)) > \s
--------------
/home/przemek/sandboxes/percona5.7.18/bin/mysql Ver 14.14 Distrib 5.7.18-15, for Linux (x86_64) using 6.3

Connection id: 343
Current database:
Current user: msandbox@localhost
SSL: Cipher in use is ECDHE-RSA-AES128-GCM-SHA256
Current pager: stdout
Using outfile: ''
Using delimiter: ;
Server version: 5.7.18-15 Percona Server (GPL), Release 15, Revision bff2cd9
Protocol version: 10
Connection: 127.0.0.1 via TCP/IP
Server characterset: latin1
Db characterset: latin1
Client characterset: utf8
Conn. characterset: utf8
TCP port: 2333
Uptime: 4 days 54 min 33 sec

Threads: 1 Questions: 1069 Slow queries: 0 Opens: 173 Flush tables: 1 Open tables: 91 Queries per second avg: 0.003
--------------

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-2283