Clarify that being able to approve changes doesn't make a user a code owner

Code owner approvals are applied by voting on a label. Which label
counts as a code owner approval is configurable. Sometimes users think
that being able to vote on that label makes them a code owner. Having
this permission is not sufficient, but to be considered as a code owner
users must be listed as a code owner in an OWNERS file. Add a note about
this to the documentation.

Release-Notes: skip
Change-Id: I39df35bb14ae81b6f6f2af69ed146a943924041d
Signed-off-by: Edwin Kempin <ekempin@google.com>
Reviewed-on: https://gerrit-review.googlesource.com/c/plugins/code-owners/+/426423
Tested-by: Zuul <zuul-63@gerritcodereview-ci.iam.gserviceaccount.com>
Reviewed-by: Patrick Hiesel <hiesel@google.com>

resources/Documentation/user-guide.md

diff --git a/resources/Documentation/user-guide.md b/resources/Documentation/user-guide.md
index a176398..1b3d4d6 100644
--- a/resources/Documentation/user-guide.md
+++ b/resources/Documentation/user-guide.md
@@ -30,6 +30,12 @@
 means it's possible that a user is a code owner, but misses permissions to apply
 code owner approvals.
 
+**NOTE**: Vice versa, having the permissions to vote on the label that is
+required as a [code owner approval](#codeOwnerApproval) doesn't make the user a
+code owner. Only users that are listed as a code owner in a relevant [code
+owner config file](#codeOwnerConfigFiles) (e.g. `OWNERS` file) are considered as
+code owner.
+
 ## <a id="whyCodeOwners">Why should code owners be used?
 
 Code owners are gatekeepers before a change is submitted, they enforce standards