fix(webhook): explain why webhook edit needs an authorization choice

Gitea 28 drops a webhook's stored authorization header on every edit
that does not send it again, and never returns it, so tea cannot leave
it unchanged. The error offered --clear-authorization as an equal
alternative, which wiped credentials users still needed. It now says
the header is lost unless sent again, asks for it first, and reserves
--clear-authorization for hooks without one.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/validation_test.go

@@ -233,7 +233,7 @@ func TestUsageErrorsPrecedeRequests(t *testing.T) {
 		{[]string{"webhook", "create", "https://host/path", "--authorization-header", "first\nsecond"}, "authorization header must be a single line"},
 		{[]string{"webhook", "create", "https://host/path", "--events", ""}, "--events needs at least one trigger"},
 		{[]string{"webhook", "create", "https://host/path", "--config", "secret=private"}, "--config cannot override the URL, secret, or webhook scope"},
-		{[]string{"webhook", "edit", "1", "--active=false"}, "editing requires --authorization-header, --authorization-header-file, or --clear-authorization because Gitea replaces authorization"},
+		{[]string{"webhook", "edit", "1", "--active=false"}, "editing a webhook drops its stored authorization header unless it is sent again, and Gitea never returns it: pass --authorization-header-file or --authorization-header (use --clear-authorization only for a hook without one)"},
 		{[]string{"webhook", "delete", "not-an-id"}, "invalid resource number \"not-an-id\""},
 		{[]string{"webhook", "list", "--org", "organization", "--account"}, "--org, --account, --global, and --repo select mutually exclusive scopes"},
 		{[]string{"webhook", "list", "--global", "-R", "owner/repo"}, "--org, --account, --global, and --repo select mutually exclusive scopes"},