doc says that execroot is the working directory for all actions, I think it's execroot/_main

### Page link:

https://bazel.build/remote/output-directories

### Problem description (include actual vs expected text, if applicable):

The page describes execroot as "The working directory for all actions". From looking at the actual arguments of actions, I think that the actual working directory is `execroot/_main`.

### Where do you see this issue? (include link to specific section of the page, if applicable)

https://bazel.build/remote/output-directories#layout-diagram

### Any other information you'd like to share?

_No response_

Open the linked output-directories page and inspect the layout diagram section. Verify the working directory used by actions against the page's description, then update the wording if the actions run from execroot/_main rather than execroot. Done means the documentation accurately matches the observed action arguments.

Written by the indexing model from the issue text.