Add 'description' field to the 'maintenance_info.'

[waterlink]

This tackles the There SHOULD be some indication of the impact of an update part of #641.

This is not necessarily about a description string containing versions changes only, this may contain details about configuration changes, the default value changes, etc.

The main use case for maintenance_info and description field is for the plans that need maintenance done to them every 2-4 weeks or so, thus creating a new plan every time is not an option.

#165560405

What is the problem this PR solves? A description of what the problem is, or a link to an issue which this is related to.

Checklist:

• [x] The swagger.yaml doc has been updated with any required changes
• [x] The openapi.yaml doc has been updated with any required changes

[cfdreddbot]

:white_check_mark: Hey waterlink! The commit authors and yourself have already signed the CLA.

[mattmcneeney]

@waterlink there is a conflict with spec.md

[waterlink]

@mattmcneeney conflict should be fixed

[jberkhahn]

This looks good to me.