[Doc] Document amazon_bedrock_config.instance_profile_arn in databricks_model_serving (#4549)

## Changes
<!-- Summary of your changes that are easy to understand -->

Resolves #4218

## Tests
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- [ ] `make test` run locally
- [x] relevant change in `docs/` folder
- [ ] covered with integration tests in `internal/acceptance`
- [ ] using Go SDK
- [ ] using TF Plugin Framework

Author: alexott

NEXT_CHANGELOG.md

@@ -10,6 +10,7 @@
 
 ### Documentation
 
+ * Document `amazon_bedrock_config.instance_profile_arn` in `databricks_model_serving` ([#4549](https://github.com/databricks/terraform-provider-databricks/pull/4549))
  * Document management of permissions of `databricks_budget_policy` resource ([#4561](https://github.com/databricks/terraform-provider-databricks/pull/4561))
  * Document `budget_policy_id` in `databricks_app` resource and data sources ([#4557](https://github.com/databricks/terraform-provider-databricks/pull/4557))
  * Add a note on how `databricks_grants` work with `MANAGE` permission [#4546](https://github.com/databricks/terraform-provider-databricks/pull/4546)

docs/resources/model_serving.md

@@ -77,6 +77,7 @@ The following arguments are supported:
       * `aws_secret_access_key` - The Databricks secret key reference for an AWS Secret Access Key paired with the access key ID, with permissions to interact with Bedrock services.
       * `aws_secret_access_key_plaintext` -  An AWS secret access key paired with the access key ID, with permissions to interact with Bedrock services provided as a plaintext string.
       * `bedrock_provider` - The underlying provider in Amazon Bedrock. Supported values (case insensitive) include: `Anthropic`, `Cohere`, `AI21Labs`, `Amazon`.
+      * `instance_profile_arn` - Optional ARN of the instance profile that the external model will use to access AWS resources. You must authenticate using an instance profile or access keys.
     * `cohere_config` - Cohere Config
       * `cohere_api_key` - The Databricks secret key reference for a Cohere API key.
       * `cohere_api_key_plaintext` - The Cohere API key provided as a plaintext string.