[gherkin-perl] Add module documentation for public API

[ehuelsmann]

Summary

Add missing API documentation.

Details

Now that the Gherkin/Perl modules are on target (i.e. based on Cucumber::Messages), we'd like people to actually be able to use these modules. Documentation is paramount to that goal.

Motivation and Context

Why, if we don't want to document the modules, would we even go through the trouble publishing them?

How Has This Been Tested?

The documentation itself has not; the tests have been run to verify no new regressions.

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• The change has been ported to Java.
• The change has been ported to Ruby.
• The change has been ported to JavaScript.
• The change has been ported to Go.
• The change has been ported to .NET.
• I've added tests for my code.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have updated the CHANGELOG accordingly.

[aurelien-reeves]

Very nice! 🤩

We should consider documenting all implementations this way!

[ehuelsmann]

Thanks for the review!