Python: document region setup for Bedrock

[eavanvalkenburg]

No description provided.

[alliscode]

@eavanvalkenburg is this issue for yourself? I've assigned to you for now.

[TaoChenOSU]

We already have a README on how to set up the bedrock service, including region. Please provide more details on this task @eavanvalkenburg, thanks!

[eavanvalkenburg]

The reason I created this issue is because I got an error that Region wasn't set, but then when I look at the init signature of the Bedrock Chat Completion there is no way to set the region, so either we add a note on that in the init or a link to the Readme in there, and I think it also helps to write out the region setup directly in the readme, rather then just having a link, I do see that region_name is a supported arg in the underlying client, so adding Kwargs to BedrockChatCompletion with a pointer to the boto client would also help, as well as adding region to the settings of Bedrock, since it doesn't work without it and it is often something that should be driven by environment settings.