codec_aom/avm.c: Add comments about useAllIntra

src/codec_aom.c

diff --git a/src/codec_aom.c b/src/codec_aom.c
index 65ae15e..f7bd620 100644
--- a/src/codec_aom.c
+++ b/src/codec_aom.c
@@ -44,6 +44,7 @@
 #endif
 
 // Speeds 7-9 were added to all intra mode in https://aomedia-review.googlesource.com/c/aom/+/140624.
+// They first appeared in the libaom v3.2.0 release.
 #if AOM_ENCODER_ABI_VERSION >= (10 + AOM_CODEC_ABI_VERSION + /*AOM_EXT_PART_ABI_VERSION=*/1)
 #define ALL_INTRA_HAS_SPEEDS_7_TO_9 1
 #endif
@@ -664,6 +665,9 @@
         codec->internal->qualityFirstLayer = quality;
     }
 
+    // Determine whether the encoder should be configured to use intra frames only, either by setting aomUsage to AOM_USAGE_ALL_INTRA,
+    // or by manually configuring the encoder so all frames will be key frames (if AOM_USAGE_ALL_INTRA isn't available).
+
     // All-intra encoding is beneficial when encoding a two-layer image item and the quality of the first layer is very low.
     // Switching to all-intra encoding comes with the following benefits:
     // - The first layer will be smaller than the second layer (which is often not the case with inter encoding)

src/codec_avm.c

diff --git a/src/codec_avm.c b/src/codec_avm.c
index 0be97be..2f7ad73 100644
--- a/src/codec_avm.c
+++ b/src/codec_avm.c
@@ -551,6 +551,9 @@
             cfg->g_limit = 1;
         }
 
+        // Determine whether the encoder should be configured to use intra frames only, by manually configuring the encoder so all
+        // frames will be key frames.
+
         // All-intra encoding is beneficial when encoding a two-layer image item and the quality of the first layer is very low.
         // Switching to all-intra encoding comes with the following benefits:
         // - The first layer will be smaller than the second layer (which is often not the case with inter encoding)
@@ -560,8 +563,8 @@
         avifBool useAllIntra = (addImageFlags & AVIF_ADD_IMAGE_FLAG_SINGLE) || useAllIntraForLayered;
 
         if (useAllIntra) {
-            // Use the default settings of the new AVM_USAGE_ALL_INTRA (added in
-            // https://crbug.com/aomedia/2959).
+            // Use the default settings of libaom's AOM_USAGE_ALL_INTRA (added
+            // in https://crbug.com/aomedia/2959).
             //
             // Set g_lag_in_frames to 0 to reduce the number of frame buffers
             // (from 20 to 2) in libavm's lookahead structure. This reduces