chore: add missing to_additive docstrings in Combinatorics and Dynamics

[adomani]

Adds the missing additive doc-strings (multiplicative side has a hand-written doc-string, additive side did not) across 5 file(s) in Combinatorics and Dynamics.

The additive doc-strings are simple-minded translations of the multiplicative ones, with referenced lemma names replaced by their to_additive counterparts. These gaps were found by an environment linter that pairs each declaration with its to_additive counterpart and checks that both or neither is documented.

🤖 Generated with Claude Code

[github-actions]

PR summary c44884d54e

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 -- pending)

Computed after the build finishes.

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No changes to strong technical debt.

No changes to weak technical debt.

Current commit c44884d54e
Reference commit 79aee35d96

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).