stdbuf: move help strings to markdown file

[m11o]

issue: #4368

stdbuf --help outputs follow.

$ ./target/debug/stdbuf --help
Run COMMAND, with modified buffering operations for its standard streams.

Mandatory arguments to long options are mandatory for short options too.

Usage: ./target/debug/stdbuf [OPTION]... COMMAND

Options:
  -i, --input <MODE>   adjust standard input stream buffering
  -o, --output <MODE>  adjust standard output stream buffering
  -e, --error <MODE>   adjust standard error stream buffering
  -h, --help           Print help
  -V, --version        Print version

If MODE is 'L' the corresponding stream will be line buffered.
This option is invalid with standard input.

If MODE is '0' the corresponding stream will be unbuffered.

Otherwise, MODE is a number which may be followed by one of the following:

KB 1000, K 1024, MB 1000*1000, M 1024*1024, and so on for G, T, P, E, Z, Y.
In this case the corresponding stream will be fully buffered with the buffer size set to MODE bytes.

NOTE: If COMMAND adjusts the buffering of its standard streams ('tee' does for e.g.) then that will override corresponding settings changed by 'stdbuf'.
Also some filters (like 'dd' and 'cat' etc.) don't use streams for I/O, and are thus unaffected by 'stdbuf' settings.

[github-actions]

GNU testsuite comparison:

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