docs: improve branchConcurrentLimit documentation

[HonkingGoose]

Changes:

• Explain that raising branchConcurrentLimit impacts Renovate's rebase performance
• Explain that config:base limits the amount of open branches at once
• Add advice from @rarkins Rebasing incredibly slow #11866 (reply in thread)
• Use one sentence per line pattern

Context:

I saw somebody getting a bit confused (#11866) on why Renovate takes so long to rebase their branches, so that's a sign that we should improve our documentation and explain.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please tick one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[renovate-release]

🎉 This PR is included in version 27.19.3 🎉

The release is available on:

• GitHub release
• 27.19.3

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