chore(Tactic): rewrite subsingleton tactic docstring

[Vierkantor]

This PR rewrites the docstrings for the subsingleton tactic, to consistently match the official style guide, to make sure they are complete while not getting too long.

The remark about Sort _ being set to Sort 0 by apply Subsingleton.elim does not seem to be true anymore, so I removed it.

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[github-actions]

PR summary b80f227194

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/pr_summary/declarations_diff.sh <optional_commit>

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

The doc-module for scripts/pr_summary/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/reporting/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).