doc: implicit fs callbacks don't throw in v0.10

But they will in v0.12.

Re #5005.
fs: Missing cb errors are deprecated, not a throw

Commit a804347 makes fs function rethrow errors when the callback is
omitted. While the right thing to do, it's a change from the old v0.8
behavior where such errors were silently ignored.

To give users time to upgrade, temporarily disable that and replace it
with a function that warns once about the deprecated behavior.

Close #5005