Moves the string instance type tag definitions around to be in order, makes
their definitions slightly clearer (in terms of shifted 1 rather than hex
values), and unifies/fixes the descriptions of the bits in the comments.

Bug: v8:8834
Change-Id: I632b93053734445264f6c607c541e4171aae8038
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1539583Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60466}