doc Document that IBM i does not support API fs.watch()

[dmabupt]

IBM i does not support the API fs.watch() -- https://github.com/nodejs/node/pull/30819/files#diff-f57f2ac1ac1926071a04f498c9fe355dR4-R6

Please add the description to the API document -- https://nodejs.org/docs/latest/api/fs.html#fs_availability

For example, On IBM i systems, this feature is not supported. Please use fs.watchFile() instead.

[richardlau]

@dmabupt Please raise a pull request in this repository with the appropriate changes to https://github.com/nodejs/node/blob/master/doc/api/fs.md.

[ThePrez]

Related to #29894 (but the end result was not explicit about IBM i)

@richardlau assign this one to me as well. I'll update the docs shortly