doc: add missing URL argument types in fs.md

Checklist

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

Affected core subsystem(s)

doc, fs

Refs:

fs.appendFile() -> fs.writeFile() -> fs.open() -> getPathFromURL()

fs.lchmod() ->fs.open() -> getPathFromURL()

fs.lchown() ->fs.open() -> getPathFromURL()

fs.truncate() ->fs.open() -> getPathFromURL()

fs.writeFile()-> fs.open() -> getPathFromURL()

fs.unwatchFile() -> getPathFromURL()