doc: clarify ambiguous rdev description

Replace "is considered 'special'" with "represents a device". The latter
is more clear and is derived from the stat() man page. The former is
also derived from the man page, but is ambiguous because:

* "considered" introduces doubt. Is it, or isn't it?
* "special" is never defined. "represents a device" communicates more
  meaning.
* "special" is in scare-quotes, introducing more doubt as to what
  exactly it means.

Refs: https://man7.org/linux/man-pages/man2/fstat.2.html

PR-URL: #34094
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ujjwal Sharma <ryzokuken@disroot.org>

Author: Trott

doc/api/fs.md

@@ -888,7 +888,7 @@ The numeric group identifier of the group that owns the file (POSIX).
 
 * {number|bigint}
 
-A numeric device identifier if the file is considered "special".
+A numeric device identifier if the file represents a device.
 
 ### `stats.size`