docs: document callout completion toggle

[unworthyzeus]

Summary

• Document the existing callout_completions setting in both configuration references.
• Show the exact callout_completions = false snippet for users who want snippets, such as LuaSnip, to handle callouts instead.
• Clarify that disabling callout completions leaves the other completion providers enabled.

Validation

• cargo fmt --check

Closes #226
/claim #226