Summary:
What existing problem does the pull request solve?
My motivation was to improve the documentation for ListView and ListViewDataSource because I always have trouble remembering how they work and the documentation was lacking an example for `cloneWithRowsAndSections`
My PR does the following:
* add a note about how headers and footers are rendered on a horizontal ListView
* fix a typo in an example
* clarify that the sectionIdentities should match the keys or array indexes of the data source
* add an example of cloneWithRowsAndSections
* add notes on the return values of `getRowCount` and `getRowAndSectionCount`
n/a; documentation only
Closes https://github.com/facebook/react-native/pull/13811
Differential Revision: D5473192
Pulled By: hramos
fbshipit-source-id: f31fc6edfdeb8dff75d39bbcceda06fd839df934
