Update the license docs

What does this MR do?

Update the license docs to reflect the current behavior of GitLab.

From the discussion in the chat:

• https://gitlab.slack.com/archives/C0NFPSFA8/p1538097929000100
• https://gitlab.slack.com/archives/C0AR2KW4B/p1535987984000100

Questions to be answered:

1. if the licence is removed from the instance, will it behave as CE then? => Yes
2. If a GitLab license expires, can product still be used as CE, or "you will not be able to push any commits to GitLab and creation of new issues and merge requests will be disabled" => Limited availability, locked features (push, issues creation, etc.)
3. Do we still give 2 week grace after expiration? https://about.gitlab.com/pricing says, “14 days after the subscription has ended, GitLab will display a notice to all users informing them of the expiration, and pushing code and creation of issues and merge requests will be disabled.”

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards