[Doc] Add bedrock multimodal default pre/post process functions

[ylwu-amzn]

We have built-in pre-process function for bedrock multi-modal model connector.pre_process.bedrock.multimodal_embedding since OS 2.16
Post process function: connector.post_process.bedrock.embedding

Need to tune current doc to add these.

1. titan Multimodal blueprint - https://github.com/opensearch-project/ml-commons/blob/main/docs/remote_inference_b[…]rints/bedrock_connector_titan_multimodal_embedding_blueprint.md
2. Supported connectors table is missing titan multimodal embeddings