Add a note about arguments after '--'

[yruellan]

Note: Arguments after '--' are treated as filenames even if they start with '-'.

Fix the issue 1926

The result is now :

$ delta -help

Usage: delta [OPTIONS] [MINUS_FILE] [PLUS_FILE]

Arguments:
  [MINUS_FILE]
          First file to be compared when delta is being used to diff two files.

          `delta file1 file2` is equivalent to `diff -u file1 file2 | delta`.

  [PLUS_FILE]
          Second file to be compared when delta is being used to diff two files.

          Note: Arguments after '--' are treated as filenames even if they start with '-'.

...

The delta -h command is unchanged because it needs to be concise.