doc: Add note about instant_restore_retention_days

[jaredbrogan]

This is required per the note presented when attempting to create a weekly backup policy in the portal.
See below:

Otherwise, this ever-present vague error will be presented (taken from activity log):

"statusMessage": "{\"error\":{\"code\":\"BMSUserErrorInvalidPolicyInput\",\"message\":\"Input for create or update policy is not in proper format. Please check format of parameters like schedule time, schedule days, retention time and retention days \"}}",

This applies to both V1 and V2 policies.

[katbyte]

Thanks for this @jaredbrogan - LGTM 📄

[github-actions]

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