Include notes about index mutation in span(after/before:) (+ other doc fixes)

[natecook1000]

The indexes passed to span(after:) and friends on UniqueArray and RigidArray are modified before return. This adds notes about the updated values to the method documentation, along with a few other minor formatting changes (dropping smart quotes, single-sentence abstracts).

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.