docs: clarify revalidateTag profile expire semantics (#97836)
Once you call `revalidateTag`, the next request that reads the tagged
data kicks off revalidation.
That tagged data is SWR until the revalidation completes, or until the
expire time, whichever comes first.
It is the profile's expire time, counted from the `revalidateTag` call,
that defines the limit after which requests block.
If the first request after `revalidateTag` is called arrives after the
expire time, it'd block.
So the profile is a way to say, after this tag is revalidated, past the
expire time, data correctness is more important than being fast.
