expand key_source documentation to include example

[ixcat]

key source is documented abstractly, would be good to provide a usage example

[kabilar]

Great work with the key_source explanation. Two minor points noted below. Thank you.

• The example described in the Custom key source section is missing.
• Additionally, the text below is duplicated: A custom key source can be configured by setting the key_source property within a table class, after the definition string.