Make the point of reusing source distribution lighter

Split the reuse of source distribution into two parts: the easier part
of building a wheel from sdist (which is what build tool does),
and the harder part of using it in all workflows. For the latter,
suggest it's fine to let downstreams worry about that.

Author: mgorny

source/discussions/downstream-packaging.rst

@@ -115,12 +115,16 @@ source archive for downstream use, for example by attaching it to a GitHub
 release. Alternatively, large files, such as test data, can be split into
 separate archives.
 
-A good idea is to **use your source distribution in the release workflow**.
-That is, build it first, then unpack it and perform all the remaining steps
-using the unpacked distribution rather than the Git repository — run tests,
-build documentation, build wheels. This ensures that it is well-tested,
-and reduces the risk that some users would hit build failures or install
-an incomplete package.
+A good idea is to use your source distribution in the release workflow.
+For example, the :ref:`build` tool does exactly that — it first builds a source
+distribution, and then uses it to build a wheel. This ensures that the source
+distribution actually works, and that it won't accidentally install fewer files
+than the official wheels.
+
+Ideally, use the source distribution also run tests, build documentation,
+and so on, or add specific tests to make sure that all necessary files were
+actually included. Understandably, this requires more effort, so it's fine
+not do that — downstream packagers will report any missing files promptly.
 
 
 .. _no-internet-access-in-builds: