docs: Update comment for omitting Sphinx 7

tox.ini

@@ -14,13 +14,12 @@ commands =
     coverage run --append --source=ocdsextensionregistry -m pytest
 deps =
     coverage
-    # ocds-babel==0.3.5 constrains mdformat>=0.7.11, yet tox installs ocds-babel==0.3.5 with mdformat==0.6.0.
     sphinx4: Sphinx>=4,<5
     sphinx5: Sphinx>=5,<6
     sphinx6: Sphinx>=6,<7
     sphinx{5,6}: docutils<0.19
-    # myst-parser>=2 is required for Sphinx>=7, but myst-parser>=2 uses markdown-it-py>=3.
-    # https://github.com/executablebooks/mdformat/issues/403
+    # Avoid DeprecationWarning from MyST-Parser until > 2.0.0. (myst-parser>=2 is required for Sphinx>=7.)
+    # https://github.com/executablebooks/MyST-Parser/issues/612
     # sphinx7: Sphinx>=7,<8
 depends =
     py-sphinx{4,5,6}: clean