Clarifying how gitlab_com works

Please provide a brief explanation for this change:

There was some confusion about how gitlab_com works. It turns out that you can leave it out of your YAML file, and it works just as well as setting it to true.

While this information was already on the page, I wanted to try to make it easier to find, so I pared down the possibilities and made them parallel.

Slack thread with some confusion was here: https://gitlab.slack.com/archives/CCB575BGT/p1655228165208469

Please indicate the types of revisions being suggested for the Product Handbook (please check all that apply):

• Small improvement (typos, clarifications, etc.)
• Adding a new section
• Modifying existing section
• Documenting a new process
• Adding a new page or directory
• Other

Please indicate Milestone

• Assigned to Milestone

Author Checklist

• Provided a concise title for the MR
• Provided a brief explanation for this change (Say why, not just what)
  • (Attach screenshots, Slack conversations, etc. as necessary)
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  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section on the page being edited.
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  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.