fs: add consistent flag fall throughs

stringToFlags() has fall throughs in a case statement.
However, they are not consistently implemented. This commit adds
consistency.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>
doc: clarify `end` vs `finish` in streams

Adds a section to the transform stream docs to clarify the
difference between the `end` event and the  `finish` events.