Replace AOM_EXT_PART_ABI_VERSION with old value

In the definition of AOM_ENCODER_ABI_VERSION, replace
AOM_EXT_PART_ABI_VERSION with its value (3) in libaom v3.2.0, so as to
not break the ABI version check in aom_codec_enc_init_ver() when an
application compiled against libaom v3.2.0 passes the old value of
AOM_ENCODER_ABI_VERSION to aom_codec_enc_init_ver().

Mark the external partition API as experimental. This was announced in
the libaom v3.2.0 release notes, but I neglected to add a note to the
source code.

Bug: 3236
Change-Id: Iefbe11ebce94c1ae7ca1e5f05f434eb71824b8fa

aom/aom_encoder.h

diff --git a/aom/aom_encoder.h b/aom/aom_encoder.h
index c324c56..1dead82 100644
--- a/aom/aom_encoder.h
+++ b/aom/aom_encoder.h
@@ -35,14 +35,24 @@
 
 /*!\brief Current ABI version number
  *
+ * \hideinitializer
  * \internal
  * If this file is altered in any way that changes the ABI, this value
  * must be bumped.  Examples include, but are not limited to, changing
  * types, removing or reassigning enums, adding/removing/rearranging
  * fields to structures
+ *
+ * Note: In the definition of AOM_ENCODER_ABI_VERSION, 3 is the value of
+ * AOM_EXT_PART_ABI_VERSION in libaom v3.2.0. The old value of
+ * AOM_EXT_PART_ABI_VERSION is used so as to not break the ABI version check in
+ * aom_codec_enc_init_ver() when an application compiled against libaom v3.2.0
+ * passes the old value of AOM_ENCODER_ABI_VERSION to aom_codec_enc_init_ver().
+ * The external partition API is still experimental. When it is declared stable,
+ * we will replace 3 with AOM_EXT_PART_ABI_VERSION in the definition of
+ * AOM_ENCODER_ABI_VERSION.
  */
 #define AOM_ENCODER_ABI_VERSION \
-  (10 + AOM_CODEC_ABI_VERSION + AOM_EXT_PART_ABI_VERSION) /**<\hideinitializer*/
+  (10 + AOM_CODEC_ABI_VERSION + /*AOM_EXT_PART_ABI_VERSION=*/3)
 
 /*! \brief Encoder capabilities bitfield
  *

aom/aom_external_partition.h

diff --git a/aom/aom_external_partition.h b/aom/aom_external_partition.h
index 68d55b7..55c59a5 100644
--- a/aom/aom_external_partition.h
+++ b/aom/aom_external_partition.h
@@ -20,6 +20,10 @@
 
 /*!\file
  * \brief Provides function pointer definitions for the external partition.
+ *
+ * \note The external partition API should be considered experimental. Until the
+ * external partition API is declared stable, breaking changes may be made to
+ * this API in a future libaom release.
  */
 
 /*!\brief Current ABI version number