[PP-1] Improve wording of Navigation requirements regarding Toolbar

Problem/Motivation

As noted by @oily while reviewing #3500632: Convert hook_requirements() that do not interact with install time, the description of the requirements check where both Navigation and Toolbar is installed could use a little clarification. Currently:

The Navigation module is a complete replacement for the Toolbar module and disables its functionality when both modules are installed. If you are planning to continue using Navigation module, you can uninstall the Toolbar module now.

@oily pointed out:

I think this should be disambiguated: '...and disables its functionality...' Does 'its' refer to the Navigation module or the Toolbar module. One would naturally guess the latter. But better to be absolutely explicit and state: 'The Navigation module is a complete replacement for the Toolbar module. When both modules are installed the Navigation module takes precedence and disables the Toolbar module's functionality...'.

Steps to reproduce

Proposed resolution

Improve the description to disambiguate "its".

Remaining tasks

1. Land #3500632: Convert hook_requirements() that do not interact with install time
2. Refine text.
3. Create MR for the changes.
4. UX review?

User interface changes

Description of the requirements warning would improve.

Introduced terminology

API changes

Data model changes

Release notes snippet