docs: add doc-comments to public functions in stats.rs (#702)

[christy-dev4]

Expand doc-comments on public functions in stats.rs

Summary

Closes #702

The public functions in contracts/split/src/stats.rs
(get_stats, increment, invoice_created, volume_added,
recipients_paid) had brief doc comments but none documented their parameters
or return values. This PR expands every public-function doc comment to describe
its env parameter, the semantic meaning of each numeric argument, and what the
returned Stats tuple represents, so callers and integrators can use the stats
module without reading the implementation.

What changed

• get_stats — documents the (total_invoices, total_volume, total_recipients_paid)
  tuple layout and the env parameter.
• increment — documents all four parameters and the StatsOverflow failure mode.
• invoice_created / volume_added / recipients_paid — document parameters and
  return, including the InvalidAmount guard on volume_added.

The Stats type alias doc and all function bodies are unchanged.

Verification

Documentation-only; compiles unaffected. (CI on main is red for unrelated,
pre-existing reasons documented in the other split PRs.)

closes #702

[drips-wave]

@christy-dev4 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits