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

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

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.

   Refs: https://github.com/nodejs/node/blob/a04f2f7df630427bf869b1e04040975b752973b6/lib/fs.js#L549

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.