6.18:14 for match arms appears inconsistent with expr.match.guard.next

From 6.18:14 of FLS:

A match arm is selected when its pattern matches the value of the subject expression and its match arm guard (if any) evaluates to true.

From expr.match.guard.next of The Rust Reference:

Otherwise, the next pattern, including other matches with the | operator in the same arm, is tested.

The ambiguity here is that, in FLS, pattern refers to the whole pattern (so the guard is for the whole arm), while in rustc, the guard can be invoked multiple times once for each alternative, depending on how many times an alternative in a pattern matches.

CC: @vapdrs

Start with FLS §6.18:14 and its linked definitions, then compare the wording with the Rust Reference's expr.match.guard.next and the described rustc behavior. Clarify whether the specification should describe guard evaluation for the whole arm or for each alternative, and update the relevant FLS text once the semantics are reconciled.