Add links explaining deprecations

Author: abravalheri

pkg_resources/__init__.py

@@ -2290,7 +2290,9 @@ def declare_namespace(packageName):
 
     msg = (
         "Implementing implicit namespace packages (as specified in PEP 420) "
-        "is preferred to `pkg_resources.declare_namespace`."
+        "is preferred to `pkg_resources.declare_namespace`. "
+        "See https://setuptools.pypa.io/en/latest/references/"
+        "keywords.html#keyword-namespace-packages"
     )
     warnings.warn(msg, DeprecationWarning, stacklevel=2)
 

setuptools/dist.py

@@ -280,7 +280,9 @@ def check_nsp(dist, attr, value):
             )
         msg = (
             "The namespace_packages parameter is deprecated, "
-            "consider using implicit namespaces instead (PEP 420)."
+            "consider using implicit namespaces instead (PEP 420). "
+            "See https://setuptools.pypa.io/en/latest/references/"
+            "keywords.html#keyword-namespace-packages"
         )
         warnings.warn(msg, SetuptoolsDeprecationWarning)