Add examples of setting individual wsrep_provider_option values dynamically in the KB

Check https://mariadb.com/kb/en/wsrep_provider_options/ to see that there are no examples on how to change dynamic provide options properly.

Galera's documentation is a bit better, see https://galeracluster.com/library/documentation/galera-parameters.html#cert-optimistic-pa for example, but when only individual option is set all the rest are reset to defaults, so all these are still misleading.

Please, clarify all the details about setting the dynamic provider options in the KB.