P2441 Fix "In that case"

[jensmaurer]

Replace "In that case..." with "If defined, ..."

"In that case" is ambiguous, because it might refer to any of three preceding conditions, or the combination of them.

Fix in P2441 and the existing views::join.

[timsong-cpp]

There are five "In that case"s in the current draft.

[JohelEGP]

Searching https://eel.is/c++draft/full for "in that case" shows 13 matches, 9 outside of notes.