Changed the description of OVERRIDE_HOSTNAME environment variable, to raise awareness of unforeseen side effects.

Description

The current documentation of OVERRIDE_HOSTNAME in mailserver.env doesnt reflect the fact that setting this can lead to unforeseen problems. See explanation at the bottom of this comment #4484 (comment)

When setting up DMS the first time i thought it would be better to set it to make sure all internal services use the correct hostname. When in reality its better not to set this unless you need to.

While at it i reformated the text so it looks more like the rest of the comments in the file. Now with no trailing whitespaces.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change that does improve existing functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
  I dont think this is necessary since the documentation already tells to only set this when you cant change the hostname.
• If necessary, I have added tests that prove my fix is effective or that my feature works
  No tests necessary, since this only changes a comment.
• New and existing unit tests pass locally with my changes
  No tests necessary, since this only changes a comment.
• I have added information about changes made in this PR to CHANGELOG.md