docs: snap remove dependency behavior undocumented

**Title:** Docs don't specify whether `snap remove` also removes runtime dependencies

Based on my understanding, removing a snap also removes all of its runtime dependencies, but this isn't explicitly confirmed in the documentation.

**References checked:**
- [Get started](https://snapcraft.io/docs/tutorials/get-started/#remove-a-snap) : no mention of dependency handling
- `snap remove --help` — no mention either

**Expected fix:** The docs (or `--help` output) should explicitly state whether `snap remove` removes unused runtime dependencies.

Start with the referenced Get started section at snapcraft.io/docs/tutorials/get-started/#remove-a-snap and inspect the `snap remove --help` output. Confirm the dependency behavior from the available project sources, then update the appropriate documentation or help text so the behavior is explicitly stated.

Written by the indexing model from the issue text.