MINOR: sink: add some comments about sft->appctx usage in applet handlers

MINOR: sink: add some comments about sft->appctx usage in applet handlers

There seem to be an ambiguity in the code where sft->appctx would differ
from the appctx that was assigned to it upon appctx creation.

In practise, it doesn't seem this could be happening. Adding a few notes
to come back to this later and try to see if we can remove this
ambiguity.