docs(functions): merge manual-only examples and notes into headers

Folds the examples Section 5 carried but the headers did not into each
function's `# EXAMPLE`, and moves the three lines that only looked like
examples -- the two typo-abbreviation notes and rm's /usr/bin/rm fallback --
into `# NOTES`, the label already in use.

Also corrects gi's synopsis, which omitted -l, and documents yt-dlp's
--no-embed-thumbnail in `# ARGUMENTS`.

functions/dng2avif.fish

@@ -25,6 +25,7 @@
 #
 # EXAMPLE
 #   dng2avif photo.dng
+#   dng2avif -q 85 -s 5 -i shot.dng -o out.avif
 function dng2avif --description 'Convert DNG raw to 10-bit HDR AVIF'
     set -l options (fish_opt -s h -l help)
     set -a options (fish_opt -s i -l input -r)