Update dynamic execution docs for multiplex sandboxing

[jjudd]

I noticed that the dynamic execution docs conflicted with the multiplex sandboxing docs and wanted to fix the discrepancy to avoid confusion.

Dynamic execution docs state:

Persistent workers will automatically run with sandboxing when used with dynamic execution, and cannot use multiplex workers.

Multiplex sandboxing docs state:

Once a worker supports multiplex sandboxing, the ruleset can declare this support by adding supports-multiplex-sandboxing to the execution_requirements of an action. Bazel will then use multiplex sandboxing if the --experimental_worker_multiplex_sandboxing flag is passed, or if the worker is used with dynamic execution.

If I understand correctly, the multiplex sandboxing docs are correct.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.