Fix `exclude path` parsing and update documentation.

[stefano-garzarella]

Summary of the PR

We don't want the shell to interpret the contents of exclude_path, but we want it all used as a single string for --ignore-filename-regex, so let's put it between double quotes.

Let's also update the exclude_path description to prevent multiple files from being separated by comma.

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [x] All commits in this PR are signed (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [x] All added/changed functionality has a corresponding unit/integration test.
• [x] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [x] Any newly added unsafe code is properly documented.

[stefano-garzarella]

I forgot to update the examples! I will do in v2.

[stefano-garzarella]

v2:

• update also the examples coverage_config_*json.sample
• fixed commit title length

[stefano-garzarella]

v3:

• escaped fullstops and added a path to a folder in the examples