doc: add note regarding special case of 0 stat.size

Refs: nodejs#43669

PR-URL: nodejs#43690
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: LiviaMedeiros <livia@cirno.name>

Author: dougwilson

doc/api/fs.md

@@ -6639,6 +6639,9 @@ A numeric device identifier if the file represents a device.
 
 The size of the file in bytes.
 
+If the underlying file system does not support getting the size of the file,
+this will be `0`.
+
 #### `stats.blksize`
 
 * {number|bigint}