Miscellaneous minor cleanups

include/avif/avif.h: Fix the comments for
avifEncoderSetCodecSpecificOption().

include/avif/internal.h: Change "KV" to "key/value" in comments.

src/write.c: Make avifEncoderCreate() initialize members of avifEncoder
in declaration order, for easier comparison with the avifEncoder struct
definition.
diff --git a/src/write.c b/src/write.c
index 3b8493d..412d35c 100644
--- a/src/write.c
+++ b/src/write.c
@@ -175,9 +175,9 @@
     encoder->tileRowsLog2 = 0;
     encoder->tileColsLog2 = 0;
     encoder->speed = AVIF_SPEED_DEFAULT;
-    encoder->data = avifEncoderDataCreate();
-    encoder->timescale = 1;
     encoder->keyframeInterval = 0;
+    encoder->timescale = 1;
+    encoder->data = avifEncoderDataCreate();
     encoder->csOptions = avifCodecSpecificOptionsCreate();
     return encoder;
 }