Document auto_tag behaviour more explicite (#37)

close #31

Co-authored-by: Lauris BH <lafriks@noreply.codeberg.org>
Reviewed-on: https://codeberg.org/woodpecker-plugins/plugin-docker-buildx/pulls/37
Reviewed-by: Lauris BH <lafriks@noreply.codeberg.org>

plugin/impl.go

@@ -123,7 +123,7 @@ func (p *Plugin) Validate() error {
 	}
 
 	if p.settings.Build.TagsAuto {
-		// return true if tag event or default branch
+		// we only generate tags on default branch or an tag event
 		if UseDefaultTag(
 			p.settings.Build.Ref,
 			p.settings.Build.Branch,