Subnet descriptioin should be expanded

Two things should be added in the subnet description when creating a sql mi:
1. Describe that the subnet needs to delegated to Managed instances. And specify that the delegation command is Microsoft.Sql/managedInstances because that is not written anywhere, and you only get that information if you write it incorrectly in cli and get an error.
2. That the subnet needs a routing table.

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#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: 06c9a36f-c18a-748b-559a-927c7f4988fb
* Version Independent ID: 7af90d97-d085-b8fa-4e02-d3e601a1846d
* Content: [az sql mi](https://docs.microsoft.com/en-us/cli/azure/sql/mi?view=azure-cli-latest)
* Content Source: [latest/docs-ref-autogen/sql/mi.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/master/latest/docs-ref-autogen/sql/mi.yml)
* Service: **sql-database**
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**

Start with latest/docs-ref-autogen/sql/mi.yml and the az sql mi content linked in the issue. Update the subnet description to state the Managed Instances delegation requirement and Microsoft.Sql/managedInstances command, plus the routing-table requirement; done means both prerequisites are documented.

Written by the indexing model from the issue text.