Clarify that source distribution needs only package's files

Let's add some clarification to the "complete source distribution" part
to make it clear that we're talking only of package's files, and not
requiring people to vendor all the dependencies.

Author: mgorny

source/discussions/downstream-packaging.rst

@@ -94,10 +94,19 @@ a few important reasons to provide a static archive file instead:
 How?
 ~~~~
 
-Ideally, **a source distribution archive should include all the files**
-necessary to build the package itself, run its test suite, build and install
-its documentation, and any other files that may be useful to end users, such as
-shell completions, editor support files, and so on.
+Ideally, **a source distribution archive should include all the files
+from the package's Git repository** that are necessary to build the package
+itself, run its test suite, build and install its documentation, and any other
+files that may be useful to end users, such as shell completions, editor
+support files, and so on.
+
+This point applies only to the files belonging to the package itself.
+The downstream packaging process, much like Python package managers, will
+provision the necessary Python dependencies, system tools and external
+libraries that are needed by your package and its build scripts. However,
+the files listing these dependencies (for example, ``requirements*.txt`` files)
+should also be included, to help downstreams determine the needed dependencies,
+and check for changes in them.
 
 Some projects are concerned about increasing the size of source distributions,
 or do not wish Python packaging tools to fall back to source distributions