[OrderedDictionary][doc] Update docs for merge/filter operations

[lorentey]

Resolves #145.

OrderedDictionary provides merge and filter operations that mostly come from the standard Dictionary type, and currently inherit documentation from them. Update docs to clarify the ordering of items in the result.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test