Fixes statements about "all branches"

What does this MR do?

Revises these two sections of the approval-rules page:

• https://docs.gitlab.com/ee/user/project/merge_requests/approvals/rules.html#add-an-approval-rule
• https://docs.gitlab.com/ee/user/project/merge_requests/approvals/rules.html#approvals-for-protected-branches

The descriptions of branches were a bit mangled:

• The subheading for adding an approval rule didn't say anything about selecting branches
• The subheading for adding an approval rule to protected branches included a full description of selecting which branches to protect, rather than just the subset of protected branches.

This merge request adds the branch-related info to the general add-a-rule section, and simplifies the description of what branches to pick when setting up approvals for protected branches. (It also deletes an outdated version history line.)

Related issues

Closes Review mentions of "any branch" (#333836 - closed)

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