Move alt_ref_buffer to tf_info

This is for easier maintenance.

BUG=aomedia:3144

Change-Id: I8edc84a73d7edf1d858634ba943218b5138225bc
diff --git a/doc/dev_guide/av1_encoder.dox b/doc/dev_guide/av1_encoder.dox
index 2f0c9e3..0f7e8f8 100644
--- a/doc/dev_guide/av1_encoder.dox
+++ b/doc/dev_guide/av1_encoder.dox
@@ -108,7 +108,7 @@
     - \ref AV1_PRIMARY.lap_enabled
     - \ref AV1_PRIMARY.twopass (\ref TWO_PASS)
     - \ref AV1_PRIMARY.p_rc (\ref PRIMARY_RATE_CONTROL)
-    - \ref AV1_PRIMARY.alt_ref_buffer (\ref yv12_buffer_config)
+    - \ref AV1_PRIMARY.tf_info (\ref TEMPORAL_FILTER_INFO)
 
 - \ref AV1_COMP
     - \ref AV1_COMP.oxcf (\ref AV1EncoderConfig)
@@ -328,7 +328,7 @@
 (see also \ref architecture_enc_data_structures):
 
 - \ref AV1_PRIMARY ppi (the primary compressor instance data structure)
-    - \ref AV1_PRIMARY.alt_ref_buffer (\ref yv12_buffer_config)
+    - \ref AV1_PRIMARY.tf_info (\ref TEMPORAL_FILTER_INFO)
 
 - \ref AV1_COMP cpi (the main compressor instance data structure)
     - \ref AV1_COMP.oxcf (\ref AV1EncoderConfig)
@@ -453,7 +453,7 @@
 The main entry point for temporal filtering is \ref av1_temporal_filter().
 This function returns 1 if temporal filtering is successful, otherwise 0.
 When temporal filtering is applied, the filtered frame will be held in
-the frame buffer \ref AV1_PRIMARY.alt_ref_buffer, which is the frame to be
+the output_frame, which is the frame to be
 encoded in the following encoding process.
 
 Almost all temporal filter related code is in av1/encoder/temporal_filter.c