Document the "nosandbox" execution_requirement on actions

[mmorearty]

In a comment, @ulfjack mentioned that an action can be marked as "nosandbox". This is not documented anywhere that I can find. Can you add it to the web page?

I'm pretty sure, from looking at the code, that this is a marker that can be put in the "execution_requirements" for ctx.action.run() or ctx.actions.run_shell() (or the older, deprecated ctx.action()). For each of those, under execution_requirements, it says, "information for scheduling the action. See tags for useful keys," and the word "tags" is a a (currently broken) link to https://docs.bazel.build/versions/master/be/common-definitions.html#common-attributes.

Two things:

1. Are an action's execution_requirements and a rule's tags really the same thing? If not, then they need to be treated separately in the docs.
2. Can you add nosandbox to the appropriate place in the docs, wherever that may be?

[ulfjack]

All the documented tags are copied to the corresponding actions, and from there to the corresponding spawns. I have a pending change to update the docs.

[jin]

no-sandbox is now documented through the tags link to https://docs.bazel.build/versions/master/be/common-definitions.html#common.tags