Ensure our custom pipeline documentation is complete

[dsherry]

This came up during @jeremyliweishih 's presentation in the usability blitz:

Custom pipeline, see how easy is it to follow documentation. creditg.csv binary classification. Copy-pasted example from doc in. Was confused what the parameters dictionary is or how to access stuff in it. What should the keys be, parameters for each component? Ran into “str comparison with float” bug, no idea what’s going on. In our doc we didn’t use one-hot encoder in example pipeline, which caused that issue. TODO file an issue to improve our custom pipeline doc.