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

Refs: nodejs#43669

Author: dougwilson

doc/api/fs.md

@@ -6639,6 +6639,8 @@ 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}