help: document weird behavior of uisetup()

While refactoring ui.log() API, it was really annoying that uisetup() is
actually an extsetup() the phase 1. Let's document that. I'm planning to
add another uisetup() which is called per ui instance, though I can't think
of a good name for it.

mercurial/help/internals/extensions.txt

@@ -159,7 +159,8 b' There are two callbacks to be called whe'
 then ``extsetup`` is called. This means ``extsetup`` can be useful in case
 one extension optionally depends on another extension.
 
-Both ``uisetup`` and ``extsetup`` receive a ui object::
+Both ``uisetup`` and ``extsetup`` receive a ui object with the local
+repository configuration::
 
     def uisetup(ui):
         # ...
@@ -167,6 +168,13 b' Both ``uisetup`` and ``extsetup`` receiv'
     def extsetup(ui):
         # ...
 
+Be aware that ``uisetup`` in NOT the function to configure a ``ui`` instance.
+It's called only once per process, not per ``ui`` instance. Also, any changes
+to the ``ui`` may be discarded because the ``ui`` here temporarily loaded
+local configuration. So, it's generally wrong to do `ui.setconfig()` in
+these callbacks. Notable exception is setting ``pre/post-<command>`` hooks
+and extending ``ui.__class__``.
+
 In Mercurial 1.3.1 or earlier, ``extsetup`` takes no argument.
 
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