fs: Added test for fs.watch recursive option

Added a test that verifies, for supported platforms only (currently
'Darwin') that recursive file watching ('recursive: true' option)
fires change events when any file is modified that is in the
directory hierarchy under the specified root.
doc: fs: Clarified recursive fs.watch support

Clarified why fs.watch recrusive support is only availavle on OS X
and why it is unlikely to be added for additional platforms.