bpo-34158: Documentation UTC offset update (GH-8377)

bpo-34158: Documentation UTC offset update (GH-8377)

* Documentation of UTC offset update
Since changes in the UTC offset that allows sub-minute offsets, the documentation needs update:

- "%z" format code documentation update
Karthikeyan Singaravelan commented on bugs.python.org:
Added as part of 018d353c1c8c87767d2335cd884017c2ce12e045 and a fix regarding duplicate words for that part was added at bac2d5ba30339298db7d4caa9c8cd31d807cf081.
Relevant format string at https://github.com/python/cpython/pull/2896/files#diff-25e2d173c84057d069b7890450714eddR214.
Relevant test case with 6-digit string for microsecond : https://github.com/python/cpython/pull/2896/files#diff-acc40bec51c7de832de3361db3edae52R309.
Table at https://docs.python.org/3.7/library/datetime.html#strftime-and-strptime-behavior could also be updated with microseconds being optional in the second column
%z | UTC offset in the form ±HHMM[SS] (empty string if the object is naive). | (empty), +0000, -0400, +1030

- isoformat documentation update
According to me, needs confirmation:
Relevant format string at https://github.com/python/cpython/pull/4699/files#diff-25e2d173c84057d069b7890450714eddR176
Relevant test case at https://github.com/python/cpython/pull/4699/files#diff-25e2d173c84057d069b7890450714edd

* From Martin Panter: some style improvment;
From @pganssle: using f for fractional part of seconds in all file.