Update periods-related comments
Update periods-related comments

The phase width in a period was bumped from 6 to 7 in
https://github.com/lampepfl/dotty/commit/16671a00371df2bdbaf6ae7f51b0ec7191ce94f3
but the comment wasn't updated.

Update the comment (and another unrelated comment).

Tested:
No

som-snytt/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,lampepfl/dotty,som-snytt/dotty,lampepfl/dotty,som-snytt/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,dotty-staging/dotty,sjrd/dotty,som-snytt/dotty,sjrd/dotty,som-snytt/dotty,dotty-staging/dotty