Track Additional Comments for ModifyVolume in KEP-3751 #549

Is your feature request related to a problem?/Why is this needed

There are some final discussions around the exact wording of the Spec in #544, this issue is created to track those comments and revisit before the changes go to Beta.

Describe the solution you'd like in detail

1. Include wording in GetCapacity API to specify that SP needs to make sure capacity being compatible with all the valid mutable parameters - conversation link

Describe alternatives you've considered
N/A

Additional context
Issue in Kubernetes is in kubernetes/enhancements#3780