TaskMutex : Address review comments from Andrew Kaufman
TaskMutex : Address review comments from Andrew Kaufman

- Fix indentation in example.
- Clarify return value of `upgradeToWriter()`.
- Add further explanatory comments.
- Reorder state assignment for consistency.

GafferHQ/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,boberfly/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,boberfly/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,hradec/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,hradec/gaffer,hradec/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,appleseedhq/gaffer,boberfly/gaffer,boberfly/gaffer,hradec/gaffer,appleseedhq/gaffer,hradec/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer