client/tailscale: document ACLTestFailureSummary.User field

And justify its legacy name.

Updates #1931

Change-Id: I3eff043679bf8f046aed6e2c4fb7592fe2e66514
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

client/tailscale/acl.go

@@ -161,7 +161,12 @@ func (c *Client) ACLHuJSON(ctx context.Context) (acl *ACLHuJSON, err error) {
 // ACLTestFailureSummary specifies the JSON format sent to the
 // JavaScript client to be rendered in the HTML.
 type ACLTestFailureSummary struct {
-	User     string   `json:"user,omitempty"`
+	// User is the source ("src") value of the ACL test that failed.
+	// The name "user" is a legacy holdover from the original naming and
+	// is kept for compatibility but it may also contain any value
+	// that's valid in a ACL test "src" field.
+	User string `json:"user,omitempty"`
+
 	Errors   []string `json:"errors,omitempty"`
 	Warnings []string `json:"warnings,omitempty"`
 }