This behavior is the same as in Go: constants can be coerced to int
and whether overflow occurs depends on how big an int is, but
this surprises people sometimes, so document it again here.

Fixes #25833.

Change-Id: I557995f1a1e8e871b21004953923d16f36cb9037
Reviewed-on: https://go-review.googlesource.com/c/141378Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>