Document mps_key_interior for amcz, and mention it in the pool introduction.

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mps/manual/source/pool/amcz.rst

@@ -68,12 +68,18 @@ AMCZ interface
       method`, an :term:`is-forwarded method` and a :term:`padding
       method`.
 
-    It accepts one optional keyword argument:
+    It accepts two optional keyword arguments:
 
     * :c:macro:`MPS_KEY_CHAIN` (type :c:type:`mps_chain_t`) specifies
       the :term:`generation chain` for the pool. If not specified, the
       pool will use the arena's default chain.
 
+    * :c:macro:`MPS_KEY_INTERIOR` (type :c:type:`mps_bool_t`, default
+      ``TRUE``) specifies whether :term:`ambiguous <ambiguous
+      reference>` :term:`interior pointers` to blocks in the pool keep
+      objects alive. If this is ``FALSE``, then only :term:`client
+      pointers` keep objects alive.
+
     For example::
 
         MPS_ARGS_BEGIN(args) {

mps/manual/source/pool/intro.rst

@@ -193,6 +193,10 @@ Blocks may use :term:`in-band headers`?        yes    yes    yes    yes    yes
            just past the end of the header) is considered to be a
            reference to the block.
 
+           Pools that support internal pointers can be switched to
+           base pointers only, by setting the optional keyword
+           argument :c:macro:`MPS_KEY_INTERIOR` to ``FALSE`` when
+           calling :c:func:`mps_pool_create_k`.
 
 .. index::
    single: pool class; writing