Best Practice: Don't use the default storage account for heavy workloads

Need to add a section in our docs on best practices, if we don't have one already. Specifically, we need to mention how one should not use the default storage account (AzureWebjobsStorage, AzureWebJobsDashboard) for your own function bindings if they'll be under any kind of heavy load.

Something like:

## Avoid heavy additional usage of the default Storage Account

It is a best practice to use a separate Storage Account for your Storage bindings. When using Storage Bindings (Blob, Queues, Tables), avoid using the default storage account (`AzureWebJobsStorage` & `AzureWebJobsDashboard`) when doing any high usage workloads (> 20 executions/second). This can lead to negative impact on the performance of the host, in addition to being throttled sooner by Storage due to the shared load of your bindings and the function host. If you're seeing high latency for your Storage bindings, errors in the portal about being unable to access your file system, or missing logs, you may want to check that you're not overloading your Azure Storage account.