docs: add explicit mention of ownProps

[anand-sundaram-zocdoc]

I noticed while trying to use the graphql() function locally that to access props of the wrapped component in the props function, it's necessary to use the key ownProps like in mapStateToProps in react-redux, but this doesn't seem to be explicitly documented anywhere, so I added it.

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes) - not a new feature
• Make sure all of the significant new logic is covered by tests - no new logic, just docs

[hwillson]

Great add - thanks @anand-sundaram-zocdoc!