Fix the tagged-union pattern-matching recommendation

The union chapter recommends matching a tag and its corresponding
union field together in a single pattern when implementing a C-style
tagged union.  This suggests the idiom is safe.

But it's not.  We don't guarantee the order that subpatterns are
checked -- this could only be safe if we could be sure the tag is
checked first.  And, in fact, if an or-pattern is used when matching
the tag, then `rustc` defers the check, causing actual UB (see
rust-lang/rust#158387).

Let's fix that.  We can remove the `items.union.pattern.subpattern`
rule entirely -- it's trivially true that a pattern can appear
as a subpattern.  In its place, let's add a warning admonition
that highlights what isn't guaranteed and describes what people
should do instead.