RELNOTES: dedup/reword/sort/add

[kmk3]

$ git log --reverse --pretty='* %s' master..
* README.md: remove items duplicated from RELNOTES
* RELNOTES: reword modif items for clarity
* RELNOTES: sort items by category then date added
* RELNOTES: add docs

[kmk3]

From commit dfc5593 ("README.md: remove items duplicated from RELNOTES",
2022-09-17):

README.md: remove items duplicated from RELNOTES

Added on commit 6d740d7 ("RELNOTES and README.md - existing
functionality modified for the next version", 2022-08-29).

The duplication makes it harder to maintain the RELNOTES, especially
because the contents are prone to diverge between each file.

For example, even though the commit in question (6d740d7) added the
same items to both files, the additions to RELNOTES had two typos, while
the additions to README.md had one typo.

The typos in RELNOTES were fixed by commit c5e8d5d ("typo fixes for
RELNOTES", 2022-08-29), and the typo in README.md was fixed by commit
2ac8a93 ("typo fix for README.md", 2022-08-29), both from PR #5349.

How about adding a link to the RELNOTES on README.md? Example:

For more details about the changes in the upcoming release, see
[RELNOTES](RELNOTES).

By the way, the modif items themselves that were added to the RELNOTES were
good; I wasn't sure about the best way to list them since they all relate to
the same issue (#5190) and because it wasn't immediately obvious the extent of
the changes done on #5190.

[glitsj16]

LGTM

[netblue30]

merged!