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/dusize.fish

@@ -16,6 +16,7 @@
 #
 # EXAMPLE
 #   dusize ~/Downloads
+#   dusize ~/Videos
 function dusize --wraps='du' --description 'alias dusize=du'
     du -sh (test -n "$argv[1]"; and echo $argv[1]; or echo .)
 end