Backport of docs: clarify capacity_min/max for volumes into release/1.2.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #12825 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

The below text is copied from the body of the original PR.

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Fixes #12821

The capacity fields for create volume set bounds on the resulting
size of the volume, but the ultimate size of the volume will be
determined by the storage provider (between the min and max). Clarify
this in the documentation and provide a suggestion for how to set a
exact size.

[github-actions]

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