Rename reflection comment to documentation

[nicolasstucki]

The name of the concept does not suggest that these are the documentation string.
It also suggests that the comments can be accessed which is not the case.

Based on #9818

[nicolasstucki]

We still use the wrong name internally in the compiler, which we could later change if we want. The important part is to have ha clear user-facing API.

[nicolasstucki]

In case the distinction between a comment and documentation is not clear

/* this is comment */
/** this is documentation */
def foo: Unit =
  // this is a comment 
  ()

[abgruszecki]

LGTM!