Document Basic Admin

[kennethpjdyer]

We have a few high-level guides scattered through the docs on basic Galera admin. For instance, my.cnf configuration, difference between setting MySQL parameters and wsrep options, as well as how to use status variables and how to query current variable settings from the client.

For the most part, these guides are deep in the Reference chapter or mentioned only in passing, rather than higher up where we actually talk about their use. Add a few short pages at the start of the Administration chapter covering these. Also, add a similar page on functions to the 4.x branch.