doc: clarify fs.write[Sync]() descriptions

1. All default values for optional `encoding` parameters
   were documented except for the one in `fs.write(fd, string...)`
   method. This PR makes up this deficiency.

2. We have two variants of `fs.write()` / `fs.writeSync()` methods:
   for buffers and strings. Currently, the sync methods have only one
   common reference to the full description of async methods.
   However, the link may seem to belong to the last sync variant only
   (for strings) and, as it refers to the first async variant
   (for buffers), this may be confusing. This PR makes two different
   sync variants refer to two different async variants.

3. In passing, both returned values of sync methods were also made
   more concise and unambiguous.

PR-URL: https://github.com/nodejs/node/pull/22402
Refs: https://github.com/nodejs/node/blob/a04f2f7df630427bf869b1e04040975b752973b6/lib/fs.js#L549


Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjami...