[diff.cpp20.library] is missing

[jensmaurer]

Look at [diff.cpp17.library], [diff.cpp14.library], [diff.cpp03.library]

We need a similar section [diff.cpp20.library] to announce new headers and removed headers.
Do we have a list somewhere?

@tkoeppe , @jwakely FYI

[frederick-vs-ja]

New headers: <expected>, <spanstream>, and <stacktrace>.
Removed headers: none.

(I wish <strstream> were removed.)

[jwakely]

And <stdatomic.h>.