[Merged by Bors] - chore: add a few missing codeblock fences to doc-strings

[grunweg]

Found by the linter in #27897, but I expect the move to Verso would also have discovered this.

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[grunweg]

@Vierkantor Here's an easy PR reformatting a few doc-strings to (presumably) also become more verso-compliant. Do you mind taking a look? Thanks!

[github-actions]

PR summary 424dce2f30

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

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No changes to strong technical debt.

No changes to weak technical debt.

[Vierkantor]

Thanks!

bors r+

[mathlib-bors]

Build failed (retrying...):

• ci (staging) / Build

[mathlib-bors]

Pull request successfully merged into master.

Build succeeded:

• ci (staging) / Post-CI job
• ci (staging) / Build
• ci (staging) / Lint style
• ci (staging) / Post-Build Step