docs: Improve documentation for IDynamoDBContext's Load and LoadAsync

[ashovlin]

Description

Improve documentation for IDynamoDBContext's Load and LoadAsync highlighting that they always rely on the table's primary key and not an override index.

Motivation and Context

#1748 & DOTNET-4899 -Load looks like it can be called using an index's key (as opposed to the table's primary key) by specifying the index name

Testing

Built DynamoDB solution locally

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license