persistent-nodemap: write down the sprint conclusion

This write down the plan for turning feature that are only fast when the rust
extension exists. Future changesets will implement it.

Differential Revision: https://phab.mercurial-scm.org/D9754

mercurial/configitems.py

@@ -1266,9 +1266,21 b' coreconfigitem('
     b'usestore',
     default=True,
 )
-# Right now, the only efficient implement of the nodemap logic is in Rust, so
-# the persistent nodemap feature needs to stay experimental as long as the Rust
-# extensions are an experimental feature.
+# Right now, the only efficient implement of the nodemap logic is in Rust,
+#
+# The case was discussed that the 5.6 sprint and the following was decided for
+# feature that have an optional fast implementation (and are a performance
+# regression in the others)
+#
+#   * If the fast implementation is not available, Mercurial will refuse to
+#     access repository that requires it. Pointing to proper documentation
+#
+#   * An option exist to lift that limitation and allow repository access.
+#
+#     Such access will emit a warning unless configured not to.
+#
+#   * When sufficiently mature, the feature can be enabled by default only for
+#   installation that supports it.
 coreconfigitem(
     b'format', b'use-persistent-nodemap', default=False, experimental=True
 )