avifenc: Disambiguate usages of vertical and horizontal in syntax

apps/avifenc.c

diff --git a/apps/avifenc.c b/apps/avifenc.c
index 19e92c0..6d99ee4 100644
--- a/apps/avifenc.c
+++ b/apps/avifenc.c
@@ -94,7 +94,7 @@
     printf("    --pasp H,V                        : Add pasp property (aspect ratio). H=horizontal spacing, V=vertical spacing\n");
     printf("    --clap WN,WD,HN,HD,HON,HOD,VON,VOD: Add clap property (clean aperture). Width, Height, HOffset, VOffset (in num/denom pairs)\n");
     printf("    --irot ANGLE                      : Add irot property (rotation). [0-3], makes (90 * ANGLE) degree rotation anti-clockwise\n");
-    printf("    --imir AXIS                       : Add imir property (mirroring). 0=vertical, 1=horizontal\n");
+    printf("    --imir AXIS                       : Add imir property (mirroring). 0=vertical axis (\"left-to-right\"), 1=horizontal axis (\"top-to-bottom\")\n");
     printf("\n");
     if (avifCodecName(AVIF_CODEC_CHOICE_AOM, 0)) {
         printf("aom-specific advanced options:\n");