Bug 1836856 - [devtools] Update actors documentation. r=devtools-reviewers,bomsy

This is a long overdue update to better describe the key actors:
root, descriptor, watcher, target and target-scope actors.

Differential Revision: https://phabricator.services.mozilla.com/D181136

UltraBlame original commit: 636272515c00d724a52319f9ae55ed09a4fcb9ca

Author: marco-c