Document that parsing never modifies the shared config

The shared-configuration warning in customize.rst now has a precise
boundary worth stating: after the parse-time mutation fix, instances
can only affect each other through explicit add/remove calls, so
concurrent parsing against the shared CONSTANTS is safe.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: derek73

docs/customize.rst

@@ -429,6 +429,9 @@ When you modify the configuration, by default this will modify the behavior all
 HumanName instances. This could be a handy way to set it up for your entire
 project, but it could also lead to some unexpected behavior because changing
 the config on one instance could modify the behavior of another instance.
+Parsing itself never modifies the configuration — only your own ``add`` and
+``remove`` calls do — so the shared instance is safe to read concurrently,
+e.g. parsing names on multiple threads.
 
 .. doctest:: module config
     :options: +ELLIPSIS, +NORMALIZE_WHITESPACE