Updated Backup Policy documentation

[PacoDw]

Description

Updated backup_policy documentation explaining how we can implement at least one policy item or remove ones.

Link to any related issue(s): #222

Type of change:

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

Required Checklist:

• I have signed the MongoDB CLA
• I have read the Terraform contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirments
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

[themantissa]

This helped me a great deal, as did running through the examples! I've included quite a bit here that I think will further clear this up. Let me know what you think.

[themantissa]

The examples are good but some of the placement of the provided info is not in the best place. Let me know if this makes sense and I can perhaps submit a PR with the changes if that will help.
Just FYI - I don't want this to hold up 0.6.2 but also want to get better examples out - we may need to do 0.6.2 first and then do a special push for just this after.

[themantissa]

@PacoDw this is a little non-standard but I made the changes I think we need in the last commit. Let me know if they make sense and then I can approve so we can move forward on closing these out. Thanks!

[PacoDw]

@themantissa no problem it's okay 👍

[themantissa]

LGTM (since made updates in last commit I think we still need). Let's wrap this up.

[coderGo93]

LGTM