Ruby: Apply suggestions from code review

Co-authored-by: Asger F <asgerf@github.com>

Author: RasmusWL

ruby/ql/lib/codeql/ruby/ApiGraphs.qll

@@ -793,7 +793,10 @@ module API {
       or
       pos.isAnyNamed() and
       result = Label::keywordParameter(_)
-      // TODO: needs handling of `self` ArgumentPosition
+      // Note: there is currently no API graph label for `self`.
+      // It was omitted since in practice it means going back to where you came from.
+      // For example, `base.getMethod("foo").getSelf()` would just be `base`.
+      // However, it's possible we'll need it later, for identifying `self` parameters or post-update nodes.
       // or
       // pos.isSelf() and
       // ...