Document how to use MySSLContextProvider for SSL connection in AsyncMysqlClient

[eryi]

Link to documentation of AsyncMysqlClient:
https://docs.hhvm.com/hack/reference/class/AsyncMysqlClient/connect/

Link to documentation of AsyncMysqlConnectionPool:
https://docs.hhvm.com/hack/reference/class/AsyncMysqlConnectionPool/connect/

Issues:

1. There is no explanation of how the MySSLContextProvider parameter works
2. AsyncMysqlConnectionPool is missing MySSLContextProvider entirely. It is unclear how we can use use SSL connections here.