Update of documentation for #4223

[jmorris-uk]

Description

Minor change to documentation pages to reflect:

• there is no longer a "default" process IN.DAT and
• if process is run with no input file given it no longer will look for a file called IN.DAT, it will run process --help.

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.63%. Comparing base (aefa624) to head (b8df26d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4241   +/-   ##
=======================================
  Coverage   48.63%   48.63%           
=======================================
  Files         151      151           
  Lines       29710    29710           
=======================================
  Hits        14449    14449           
  Misses      15261    15261           

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[clmould]

A bit of rearranging please (see comments)

Also, there is a reference to a default IN.DAT in input-guide.md - could this be removed as well please?