[fortran/en] Fixed overly long comment lines

[jklebes]

• I solemnly swear that this is all original content of which I am the original author
• Pull request title is prepended with [language/lang-code] (example [python/fr-fr] or [java/en])
• Pull request touches only one file (or a set of logically related files with similar changes made)
• Content changes are aimed at intermediate to experienced programmers (this is a poor format for explaining fundamental programming concepts)
• If you've changed any part of the YAML Frontmatter, make sure it is formatted according to CONTRIBUTING.md
  • Yes, I have double-checked quotes and field names!

Fixed overly long comment lines > 80 characters that overflow their line when displayed online. This includes rephrasing and moving some explanations. Content was added : one additional comment at 'subroutine'.

[vendethiel]

It's true the overflow doesn't look the best. Thanks!