join: move help strings to markdown file

[piotrkwiecinski]

#4368

join --help outputs the following:

target/debug/join --help
For each pair of input lines with identical join fields, write a line to
standard output. The default join field is the first, delimited by blanks.

When `FILE1` or `FILE2` (not both) is `-`, read standard input.

Usage: target/debug/join <FILE1> <FILE2>

Options:
  -a <FILENUM>           also print unpairable lines from file FILENUM, where
                         FILENUM is 1 or 2, corresponding to FILE1 or FILE2 [possible values: 1, 2]
  -v <FILENUM>           like -a FILENUM, but suppress joined output lines [possible values: 1, 2]
  -e <EMPTY>             replace missing input fields with EMPTY
  -i, --ignore-case      ignore differences in case when comparing fields
  -j <FIELD>             equivalent to '-1 FIELD -2 FIELD'
  -o <FORMAT>            obey FORMAT while constructing output line
  -t <CHAR>              use CHAR as input and output field separator
  -1 <FIELD>             join on this FIELD of file 1
  -2 <FIELD>             join on this FIELD of file 2
      --check-order      check that the input is correctly sorted, even if all input lines are pairable
      --nocheck-order    do not check that the input is correctly sorted
      --header           treat the first line in each file as field headers, print them without trying to pair them
  -z, --zero-terminated  line delimiter is NUL, not newline
  -h, --help             Print help
  -V, --version          Print version

[cakebaker]

Thanks!