Update the scope-when-expressions-to-task feature flag docs

In tektoncd#4580, we changed the
flag default from "false" to "true". However, the documentation
above the flag was still describing what setting it to "true" would
do. In this change, we update the documentation to focus on the
non-default option that users can choose to set - "false". We also
add a reference to TEP-0059 and relevant docs for more details.

config/config-feature-flags.yaml

@@ -82,6 +82,7 @@ data:
  # Setting this flag will determine which gated features are enabled.
  # Acceptable values are "stable" or "alpha".
  enable-api-fields: "stable"
- # Setting this flag to "true" scopes when expressions to guard a Task only
- # instead of a Task and its dependent Tasks.
+ # Setting this flag to "false" scopes when expressions to guard a Task and
+ # its dependent Tasks. This flag defaults to "true"; when expressions guard
+ # the Task only. See TEP-0059 and Pipeline documentation for more details.
  scope-when-expressions-to-task: "true"