bpo-30661: Improve doc for tarfile pax change and effect on shutil

[CAM-Gerlach]

As a followup to bpo-36268 and PR #12355 , to fully resolve bpo-30661 this PR makes the following documentation changes:

• Adds a mention of the now-default pax format to the tar shutil.make_archive format, and adds a versionchanged note to this effect to shutil.make_archive.
• Adds a mention of the change to the shutil section of the What's New document.
• Makes some minor textual fixes, clarifications and improvements to the tarfile formats section, and fixes an incorrect indent level for the versionchanged section there.

I didn't add a NEWS item for this change, but I can do so if desired (@ncoghlan ?).

https://bugs.python.org/issue30661

[vadmium]

Was doubling modern meant to distract me from noticing 2001 is eighteen years ago? :)

[CAM-Gerlach]

Heh. "modern"-est, I suppose, although to be fair it took even libarchive (modern bsdtar) 10 years to implement fully complete support (ironically, GNU tar had it far earlier). And would you believe I've worked as a freelance copyeditor?

In any case, fixed, thanks.

[ncoghlan]

Thanks! And I agree this doesn't need a separate NEWS item - it's covered by the existing one for the tarfile change, as it's just updating the docs to explain shutil inherits that change in defaults.