doc: Make it clear that requirements/constraints file can be a URL

docs/html/user_guide.rst

@@ -127,6 +127,10 @@ Logically, a Requirements file is just a list of :ref:`pip install` arguments
 placed in a file. Note that you should not rely on the items in the file being
 installed by pip in any particular order.
 
+Requirements files can also be served via a URL, e.g.
+http://example.com/requirements.txt besides as local files, so that they can
+be stored and served in a centralized place.
+
 In practice, there are 4 common uses of Requirements files:
 
 1. Requirements files are used to hold the result from :ref:`pip freeze` for the
@@ -248,6 +252,10 @@ undocumented and unsupported quirks from the previous implementation,
 and stripped constraints files down to being purely a way to specify
 global (version) limits for packages.
 
+The same as requirements files, constraints files can be served via a URL, e.g.
+http://example.com/constraints.txt, so that your organization can store and
+serve them in a centralized place.
+
 .. _`Installing from Wheels`:
 
 

news/11954.doc.rst

@@ -0,0 +1 @@
+Make it clear that constraints file can be a URL