docs: document closure-by-reference behavior and loop variable gotcha

[alexanderxfgl-bit]

Closes #5

Adds docs/language/functions.md documenting:

• Closure-by-reference behavior (same as Python/JavaScript)
• The loop variable gotcha with example
• The capture() workaround pattern
• Default parameter snapshotting alternative

[Elabsurdo984]

Hi, your pull request is correct. Apologies for not notifying you about this; currently, the documentation repository is separate from the language. If you'd like, you can submit your pull request to https://github.com/luz-lang/luzdocs. Thank you for your contribution.