Add note to upgrade guide about provider sensitivity (#26643)

* Add note to upgrade guide about provider sensitivity

Now that sensitivity follows attributes providers mark
as sensitive, add this note to the upgrade guide.

Co-authored-by: Alisdair McDiarmid <alisdair@users.noreply.github.com>

website/upgrade-guides/0-14.html.markdown

@@ -323,9 +323,9 @@ instead of the actual value.
 Terraform v0.14 introduces a more extensive version of that behavior where
 Terraform will track when you write an expression whose result is derived
 from a
-[sensitive input variable](/docs/configuration/outputs.html#sensitive-suppressing-values-in-cli-output)
-or
+[sensitive input variable](/docs/configuration/outputs.html#sensitive-suppressing-values-in-cli-output),
 [sensitive output value](/docs/configuration/variables.html#suppressing-values-in-cli-output),
+or an attribute defined as sensitive by a provider,
 and so after upgrading to Terraform v0.14 you may find that more values are
 obscured in the Terraform plan output than would have been in Terraform v0.13.