doc: cleanup and improve for recently moved pkgs (#354)

* chore(atomicx): review docs and add usage example

* chore(fsx): improve docs, return value, add examples

* fix(kvstore): correct typo and add example

* fix(multierror): add basic example

* doc: revamp ooapi documentation

CONTRIBUTING.md

@@ -57,7 +57,9 @@ run `go mod tidy` to minimize such changes.
 
 ## Implementation requirements
 
-Please, use `./internal/atomicx` rather than `atomic/sync`.
+- use `./internal/atomicx` rather than `atomic/sync`
+
+- use `./internal/fsx.OpenFile` when you need to open a file
 
 Do now use `os/exec`, use `x/sys/execabs`.