doc: update the description of the return type for options.filter

[kylo5aby]

The return type of options.filter in fs.cp, fs.cpSync, and fsPromises.cp can be coerced to types that are coercible to boolean, not only boolean, So I update related contents in the doc.

Once this PR has no further issues, I will close #52461.

Refs: #52461

[kylo5aby]

/cc @aduh95 @jasnell