doc: fix TFAE list rendering in Rees theorem module docstring

[kim-em]

This PR fixes the module docstring of Mathlib/RingTheory/Depth/Rees.lean: the four TFAE items used ·, which is not a markdown list marker, so they rendered as a single run-on paragraph on doc-gen. Replace them with proper * sub-list items, add the missing "and nontrivial" to the first item to match the Lean statement, and drop the unused binder name Nfin in subsingleton_ext_of_exists_isRegular.

Follow-up to #26212 (feat(Algebra): the Rees theorem for depth), where the reviewer deferred checking the rendered docs to after the merge.

🤖 Prepared with Claude Code

[github-actions]

PR summary c3933ae8e6

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 (regex)

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.

Declarations diff (Lean)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit c3933ae).

• +0 new declarations
• −0 removed declarations

No declaration differences.

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No changes to strong technical debt.

No changes to weak technical debt.

Current commit c3933ae8e6
Reference commit a33a5ccd63

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/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).