docs(config): state data directory behavior positively

Documentation should describe what the software does, not enumerate
what it does not do. Rephrase the data-directory wording around the
positive facts (each directory is independent; a new root starts with
empty state) and drop the "never moves or copies" constructions from
config.md, the config.html env table, and the changeset.

docs/config.html

@@ -415,7 +415,7 @@
                       <td><code>PI_WEB_DATA_DIR</code></td>
                       <td>Process/env</td>
                       <td>Not supported locally</td>
-                      <td>Restart services before changing; selects a new state location without moving existing state</td>
+                      <td>Restart services before changing; existing state stays in the previous data directory</td>
                     </tr>
                     <tr>
                       <td>Session daemon socket</td>