[Merged by Bors] - chore(Geometry/Manifold): make some doc-strings follow the style guide

[grunweg]

such as, by them beginning with the object they are defining as a subject.
This will yield much better doc-strings for the differential geometry elaborators in #39677.

It also increases conformance with the documentation style guide.

Inspired by the MI retreat in Lisbon.

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[github-actions]

PR summary 106ab480a9

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 106ab48).

• +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 106ab480a9
Reference commit 8d1ee6c7d5

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).

[grunweg]

@Vierkantor on the stylistic aspect (only if you'd like)

[mathlib-merge-conflicts]

This pull request has conflicts, please merge master and resolve them.

[PatrickMassot]

bors merge

[mathlib-bors]

Pull request successfully merged into master.

Build succeeded:

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