ptx: move help strings to markdown file

[chenchiii]

#4368

ptx -h outputs the following.

$ ./target/debug/coreutils ptx -h
Output a permuted index, including context, of the words in the input files.
Mandatory arguments to long options are mandatory for short options too.
With no FILE, or when FILE is -, read standard input. Default is '-F /'.

Usage: ./target/debug/coreutils ptx [OPTION]... [INPUT]...  
./target/debug/coreutils ptx -G [OPTION]... [INPUT [OUTPUT]]

Options:
  -A, --auto-reference            output automatically generated references
  -G, --traditional               behave more like System V 'ptx'
  -F, --flag-truncation <STRING>  use STRING for flagging line truncations
  -M, --macro-name <STRING>       macro name to use instead of 'xx'
  -O, --format=roff               generate output as roff directives
  -R, --right-side-refs           put references at right, not counted in -w
  -S, --sentence-regexp <REGEXP>  for end of lines or end of sentences
  -T, --format=tex                generate output as TeX directives
  -W, --word-regexp <REGEXP>      use REGEXP to match each keyword
  -b, --break-file <FILE>         word break characters in this FILE
  -f, --ignore-case               fold lower case to upper case for sorting
  -g, --gap-size <NUMBER>         gap size in columns between output fields
  -i, --ignore-file <FILE>        read ignore word list from FILE
  -o, --only-file <FILE>          read only word list from this FILE
  -r, --references                first field of each line is a reference
  -w, --width <NUMBER>            output width in columns, reference excluded
  -h, --help                      Print help information
  -V, --version                   Print version information

[github-actions]

GNU testsuite comparison:

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

[sylvestre]

upstream has:

       ptx - produce a permuted index of file contents

could you please add it?

[chenchiii]

Sure, I can add it for more detail.
Should I add this description in the front of the about section (Line 8)?

coreutils/src/uu/ptx/ptx.md

Line 8 in 03db304

Output a permuted index, including context, of the words in the input files.

I feel like this line also has a similar description.

[sylvestre]

@chenchiii yes please :)

[chenchiii]

@sylvestre I added it.