PEP 621: Clearly specify that PEP 621 is not meant to be read from an sdist (#1474)

Co-authored-by: Dustin Ingram <di@users.noreply.github.com>
Co-authored-by: Paul Ganssle <p.ganssle@gmail.com>

pep-0621.rst

@@ -29,14 +29,15 @@ Motivation
 The key motivators of this PEP are:
 
 - Encourage users to specify core metadata statically for speed,
-  ease of specifying, and deterministic consuming
+  ease of specification, deterministic consumption by build back-ends,
+  and ease analysis of source checkouts
 - Provide a tool-agnostic way of specifying the metadata for ease of
   learning and transitioning between build back-ends
 - Allow for more code sharing between build back-ends for the
   "boring parts" of a project's metadata
 
 This PEP does **not** attempt to standardize all possible metadata
-required to build a wheel, only the metadata covered by the
+required by a build back-end, only the metadata covered by the
 `core metadata`_ specification which are very common across projects
 and would stand to benefit from being static and consistently
 specified. This means build back-ends are still free and able to
@@ -49,6 +50,15 @@ This PEP is also not trying to change the underlying `core metadata`_
 in any way. Such considerations should be done in a separate PEP which
 may lead to changes or additions to what this PEP specifies.
 
+Finally, this PEP is meant for users to specify metadata for build
+back-ends or those doing analysis on a source checkout. Once a build
+back-end has produced an artifact, then the metadata
+contained in the artifact that the build back-end produced should be
+considered canonical and overriding what this PEP specifies. In the
+eyes of this PEP, a source distribution is considered a build
+artifact, thus people should not read the metadata specified in this
+PEP as the canonical metadata in a source distribution.
+
 
 Rationale
 =========