RFC: Clarify that the direct_url.json url field must be a spec-compliant url

Clarify that the `url` field in `direct_url.json` must be a valid url.

Author: konstin

source/specifications/direct-url-data-structure.rst

@@ -10,20 +10,16 @@ This document specifies a JSON-serializable abstract data structure that can rep
 URLs to python projects and distribution artifacts such as VCS source trees, local
 source trees, source distributions and wheels.
 
-The representation of the components of this data structure as a :rfc:`1738` URL
-is not formally specified at time of writing. A common representation is the pip URL
-format. Other examples are provided in the :ref:`Version specifier specification <version-specifiers>`.
-
-
 Specification
 =============
 
 The Direct URL Data Structure MUST be a dictionary, serializable to JSON according to
 :rfc:`8259`.
 
 It MUST contain at least two fields. The first one is ``url``, with
-type ``string``. Depending on what ``url`` refers to, the second field MUST be
-one of ``vcs_info`` (if ``url`` is a VCS reference), ``archive_info`` (if
+type ``string``. Its content must be a valid URL according to the
+`WHATWG URL Standard <whatwg-url-standard_>`_. Depending on what ``url`` refers to,
+the second field MUST be one of ``vcs_info`` (if ``url`` is a VCS reference), ``archive_info`` (if
 ``url`` is a source archives or a wheel), or ``dir_info`` (if ``url``  is a
 local directory). These info fields have a (possibly empty) subdictionary as
 value, with the possible keys defined below.
@@ -396,3 +392,4 @@ History
 
 
 .. _archive-info-hashes: https://discuss.python.org/t/22299
+.. _whatwg-url-standard: https://url.spec.whatwg.org/