ruby-on-rails-advanced-forms: Add link to docs on checkbox gotcha

[dwlknsn]

Because

Helps to clarify that a form not submitting the params for a deselected checkbox/select form field is a feature of the HTML spec, not an unintended side-effect. It's a known gotcha, but reads like a bit of an unintended side effect.

The rails API guides includes a helpful section with details of this gotcha and why a hidden field is required, including examples of the HTML output.

This PR

• adds a link to the rails API guides to clarify the hidden form fields on checkboxes

Issue

Closes #XXXXX

Additional Information

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project curriculum contributing guide
• [x] The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] If this PR addresses an open issue, it is linked in the Issue section
• [x] If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• [x] If any lesson files are included in this PR, they follow the Layout Style Guide

[dwlknsn]

@JoshDevHub - suggestion applied, thanks. Ready for review and merge 🚀