docs/library/os: Clarify ilistdir tuples size element.

The currently documentation for ilistdir tuples says that the size element
is included based on the platform. However, this is not the case as it is
included based on the filesystem type. This commit makes the according
adjustment.

Fixes issue micropython#17516.

Signed-off-by: Alex Tran <alex.t.tran@gmail.com>

Author: alectric-tr

docs/library/os.rst

@@ -55,10 +55,9 @@ Filesystem access
       directories and 0x8000 for regular files;
     - *inode* is an integer corresponding to the inode of the file, and may be 0
       for filesystems that don't have such a notion.
-    - Some platforms may return a 4-tuple that includes the entry's *size*.  For
-      file entries, *size* is an integer representing the size of the file
-      or -1 if unknown.  Its meaning is currently undefined for directory
-      entries.
+    - *size* is an integer that may be included depending on the filesystem type.
+      For file entries, *size* represents the size of the file or -1 if unknown.
+      Its meaning is currently undefined for directory entries.
 
 .. function:: listdir([dir])