Fix/Improve DistCp doc

Fix/Improve stuff in DistCp doc:

• Missing Headings from the index like (DistCp and Object Stores)
• Add about -delete along with -update in example for Object Store usage.
• Check the usage description for -update. It says it matches blocksize as well, which isn't true unless you specify -pb and if you do so, for cloud storages it would overwrite always.(AFAIK) need to double check.

And if possible other improvements