fixing off-by-one error in example multipart code

We based our system on this code but found the calculation of archive_size was
off by one.  The postincrement "$part_index++" doesn't happen if the
if the first part of the conditional fails.
s/multi-part upload/MULTIPART UPLOAD OPERATIONS/

to match the actual heading of that section, so you can find it
adding comments to MULTIPART section
fixing link in pod in last commit
improvements to POD

Fixed some typos, changed the pod to match the code, added linebreaks to make
it easier to read, wrapped at 80 columns to make it easier to proofread.