Updates to default-branch info

What does this MR do?

Addresses concerns in Write documentation for default branch names fr... (#378499 - closed) about information for updated branches. The questions I still needed to address:

1. Do the docs say that the fields Default branch and Branch name template are now listed under Branch defaults? (This location changed some time ago.)
2. Create a better, more obvious cross-link between Create a branch from an issue to the instructions further down the page, and make clear that variables are available.
3. Make sure the %{id} and %{title} variables are documented.
4. Check what tiers can create branches from issues.

So far, only one of the items actually needed a change, but I still need confirmation on what tiers can create branches from issues. I'll need to send that question over to a groupsource code engineer.

Related issues

• Closes Write documentation for default branch names fr... (#378499 - closed)
• Related to Configure branch name for creating MR from an i... (!93108 - merged)

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