avifenc: Update the default yuv format for grayscale PNG

Add a note to avifenc's help message to clarify that YUV400 will
be used in "auto" mode if the input is a grayscale PNG.

Fixes #1523

apps/avifenc.c

diff --git a/apps/avifenc.c b/apps/avifenc.c
index c98a6d5..034a3dc 100644
--- a/apps/avifenc.c
+++ b/apps/avifenc.c
@@ -184,7 +184,7 @@
     printf("    -l,--lossless                     : Set all defaults to encode losslessly, and emit warnings when settings/input don't allow for it\n");
     printf("    -d,--depth D                      : Output depth [8,10,12]. (JPEG/PNG only; For y4m or stdin, depth is retained)\n");
     printf("    -y,--yuv FORMAT                   : Output format [default=auto, 444, 422, 420, 400]. Ignored for y4m or stdin (y4m format is retained)\n");
-    printf("                                        For JPEG, auto honors the JPEG's internal format, if possible. For all other cases, auto defaults to 444\n");
+    printf("                                        For JPEG, auto honors the JPEG's internal format, if possible. For grayscale PNG, auto defaults to 400. For all other cases, auto defaults to 444\n");
     printf("    -p,--premultiply                  : Premultiply color by the alpha channel and signal this in the AVIF\n");
     printf("    --sharpyuv                        : Use sharp RGB to YUV420 conversion (if supported). Ignored for y4m or if output is not 420.\n");
     printf("    --stdin                           : Read y4m frames from stdin instead of files; no input filenames allowed, must set before offering output filename\n");