javadocs for FileInputFormat and OutputFormat to mention DT collection

the fact that DTs are collected for a job in FileInputFormat.listStatus(JobConf job) and OutputFormat.checkOutputSpecs is not something mentioned in the javadocs, or that obvious when you look @ the API.

Add a sentence to the javadocs of the relevant methods to make clear "the job you pass in is altered; anyone subclassing needs call their superclass or do something similar