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/copy.fish

@@ -17,6 +17,7 @@
 #
 # EXAMPLE
 #   copy ./mydir/ ~/backup
+#   copy ./mydir/ ~/backup    # copies mydir INTO backup, not backup/mydir/
 function copy
     set count (count $argv)
     if test "$count" = 2; and test -d "$argv[1]"