docs: Add AWS Assume Role authentication to Bedrock Chat Model and Embeddings Bedrock nodes

[rsedletsky]

Related PR

n8n-io/n8n#29717

What changed

PR #29717 adds an Authentication parameter to the AWS Bedrock Chat Model and Embeddings AWS Bedrock nodes, allowing users to choose between:

• IAM User (existing behavior, AWS access key/secret)
• Assume Role (new, uses the existing awsAssumeRole credential via STS)

Docs changes needed

1. docs/integrations/builtin/credentials/aws.md

In the AWS (Assume Role) credentials section, add both Bedrock nodes to the supported nodes list:

- [AWS Bedrock Chat Model](/integrations/builtin/cluster-nodes/sub-nodes/n8n-nodes-langchain.lmchatawsbedrock.md)
- [Embeddings AWS Bedrock](/integrations/builtin/cluster-nodes/sub-nodes/n8n-nodes-langchain.embeddingsawsbedrock.md)

2. docs/integrations/builtin/cluster-nodes/sub-nodes/n8n-nodes-langchain.lmchatawsbedrock.md

Add an Authentication entry under Node parameters:

* **Authentication**: Choose the authentication method:
    * **IAM User**: Authenticate using AWS IAM access keys (default).
    * **Assume Role**: Authenticate by assuming an IAM role using AWS STS. Requires an [AWS Assume Role credential](/integrations/builtin/credentials/aws.md). Recommended for EKS/ECS workloads using pod or task IAM roles.

Update the credentials note to reference both credential types:

/// note | Credentials
You can find authentication information for this node [here](/integrations/builtin/credentials/aws.md).
///

(This is already correct — no change needed.)

3. docs/integrations/builtin/cluster-nodes/sub-nodes/n8n-nodes-langchain.embeddingsawsbedrock.md

Same Authentication entry under Node parameters as above.