[DOC] Document limitation of userstats regarding partitioned tables

Same with documentation for 5.1 and 5.6 - we have to document this limitation until https://bugs.launchpad.net/percona-server/+bug/625045 is fixed:

[openxs@chief p5.6]$ bin/mysql --no-defaults -uroot test
Reading table information for completion of table and column names
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Server version: 5.6.12 MySQL Community Server (GPL)

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mysql> create table t1 (id int primary key) engine=InnoDB; Query OK, 0 rows affected (0.41 sec)

mysql> create table p1 (id int primary key) engine=InnoDB partition by hash(id) partitions 8;
Query OK, 0 rows affected (2.59 sec)

mysql> insert into t1 values (2), (33), (3), (100), (4);
Query OK, 5 rows affected (0.04 sec)
Records: 5 Duplicates: 0 Warnings: 0

mysql> insert into p1 values (2), (33), (3), (100), (4);
Query OK, 5 rows affected (0.03 sec)
Records: 5 Duplicates: 0 Warnings: 0

mysql> set global userstat=1;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from t1;
+-----+
| id |
+-----+
| 2 |
| 3 |
| 4 |
| 33 |
| 100 |
+-----+
5 rows in set (0.00 sec)

mysql> select * from p1;
+-----+
| id |
+-----+
| 33 |
| 2 |
| 3 |
| 4 |
| 100 |
+-----+
5 rows in set (0.00 sec)

mysql> select * from INFORMATION_SCHEMA.INDEX_STATISTICS;
+--------------+------------+------------+-----------+
| TABLE_SCHEMA | TABLE_NAME | INDEX_NAME | ROWS_READ |
+--------------+------------+------------+-----------+
| test | t1 | PRIMARY | 5 |
+--------------+------------+------------+-----------+
1 row in set (0.00 sec)

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-1406