doc(GroupTheory/Commensurable): clarify some docstrings

[kbuzzard]

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I was trying to learn about commensurators from mathlib, but all the docstrings seemed to have been written for people who already knew what the definitions were. This wasn't hard to fix.

[github-actions]

PR summary ed44d87467

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).

[adomani]

Thanks: this is much clearer!

bors d+

[adomani]

Suggested change

	@[to_additive "The index of a subgroup as a natural number,
	and returns 0 if the index is infinite."]
	@[to_additive "The index of a subgroup as a natural number.
	Returns 0 if the index is infinite."]

I'm not sure that the suggestion will work, as it is outside of the modified lines.

[mathlib-bors]

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