bpo-34273: Change 'Fixed point' to 'Fixed-point notation'. (GH-8673)

bpo-34273: Change 'Fixed point' to 'Fixed-point notation'. (GH-8673)

* bpo-34273: Change 'Fixed point' to 'Fixed-point notation'.
The change in the mini language floating point and decimal table
is consistent with 'Exponential notation' and clarifies that we
are referring to the output notation, not an object type.

* Update string.rst

* Update string.rst

* Update string.rst

* Update string.rst
(cherry picked from commit 28c7f8c8ce34a0cb848822a252a9d0a761fb42d5)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>