[FLINK-1157] Document TaskManager slots and minor fixes.

- Move some background on the most important configuration values into the configuration guide
- Remove the term "UDF" because it is often being confused with UDFs in SQL
- Fixe some typos
I think if somebody is already spending 15 minutes setting up Flink on a cluster, they should also spend 5 more minutes on configuring it properly, to get the full performance.