Clarify that the scripts directory is for files only

Following astral-sh/uv#9656 and the discussion in the pypa discord (https://discord.com/channels/803025117553754132/837243676814999553/1314391181656064040), add a clarifying sentence that the scripts directory in wheels if for files only.

A future specification may want to extend this to "regular files and symlinks", for not it's limited to regular for clear rewrite semantics.

Author: konstin

source/specifications/binary-distribution-format.rst

@@ -189,7 +189,7 @@ its version, e.g. ``1.0.0``, consist of:
 #. Python scripts must appear in ``scripts`` and begin with exactly
    ``b'#!python'`` in order to enjoy script wrapper generation and
    ``#!python`` rewriting at install time.  They may have any or no
-   extension.
+   extension.  The ``scripts`` directory may only contain regular files.
 #. ``{distribution}-{version}.dist-info/METADATA`` is Metadata version 1.1
    or greater format metadata.
 #. ``{distribution}-{version}.dist-info/WHEEL`` is metadata about the archive