[DOCS] Clarify when to use kbn clean (elastic#101155)

When building a PR locally, I ran into an issue where the server kept crashing. I ran `yarn kbn clean`, and saw this message in my terminal:
>warn This command is only necessary for the rare circumstance where you need to recover a consistent state when problems arise. If you need to run this command often, >please let us know by filling out this form: https://ela.st/yarn-kbn-clean

I think it makes sense to add this information to the docs so that if users are reading it, they know that this command is not typically necessary.

docs/developer/getting-started/index.asciidoc

@@ -92,6 +92,10 @@ may need to run:
 yarn kbn clean
 ----
 
+NOTE: Running this command is only necessary in rare circumstance where you need to recover
+a consistent state when problems arise. If you need to run this command often, complete
+this form to provide feedback: https://ela.st/yarn-kbn-clean
+
 If you have failures during `yarn kbn bootstrap` you may have some
 corrupted packages in your yarn cache which you can clean with: