Documentation usability feedback for run.md

[frankfarzan]

https://github.com/GoogleContainerTools/kpt/blob/master/docs/fn/run.md

• Caveats section is very long since it's explaining multiple concepts. It's probably worth creating subsection for each
• Need to use verbatim code style in many instances. e.g. metadata.annotations.[config.kubernetes.io/function]. Otherwise, it's hard to read.
• Config Functions -> KPT Functions ?
• We should simplify number of ways to invoke functions declarativey. Or at least, emphasize one approach in docs so user doesn't get overwhelmed.
• It's ambiguous whether functions directory is required or just a convention used (functions dir is in fact a special dir recognized by kpt).
• Also note that the implicit pattern is currently broken: kpt fn run scoping broken #188

[droot]

This will be addressed with the docs revamp (and pipeline implementation) targeted for v1.0