chore(Tactic/CongrExclamation): improve congr! docstring

[Vierkantor]

This PR (re)writes the docstring for the congr! tactic, to consistently match the official style guide, to make sure it is complete while not getting too long.

The existing docstring for congr! was very detailed, which is good for e.g. a reference manual, but as a docstring it was hard to quickly find the relevance. I moved this docstring to the module docs, and instead wrote a new docstring that does not list all those details, referring to the module docs for the full picture.

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[github-actions]

PR summary a786f33605

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

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

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).