mknod: move help strings to markdown file

[papparapa]

#4368

mknod -h outputs the following.

$ ./target/debug/coreutils mknod -h
Create the special file NAME of the given TYPE.

Usage: ./target/debug/coreutils mknod [OPTION]... NAME TYPE [MAJOR MINOR]

Arguments:
  <NAME>   name of the new file
  <TYPE>   type of the new file (b, c, u or p)
  [MAJOR]  major file type
  [MINOR]  minor file type

Options:
  -m, --mode <MODE>  set file permission bits to MODE, not a=rw - umask
  -h, --help         Print help
  -V, --version      Print version

Mandatory arguments to long options are mandatory for short options too.
-m, --mode=MODE    set file permission bits to MODE, not a=rw - umask
--help     display this help and exit
--version  output version information and exit

Both MAJOR and MINOR must be specified when TYPE is b, c, or u, and they
must be omitted when TYPE is p.  If MAJOR or MINOR begins with 0x or 0X,
it is interpreted as hexadecimal; otherwise, if it begins with 0, as octal;
otherwise, as decimal.  TYPE may be:

b      create a block (buffered) special file
c, u   create a character (unbuffered) special file
p      create a FIFO

NOTE: your shell may have its own version of mknod, which usually supersedes
the version described here.  Please refer to your shell's documentation
for details about the options it supports.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail-2/inotify-dir-recreate. tests/tail-2/inotify-dir-recreate is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/tail-2/inotify-dir-recreate is no longer failing!