Add boolean operator guidance to Elixir usage rules

[Vitaly-A-Ivanov]

Summary

Adds guidance to usage-rules/elixir.md for choosing && / || versus and / or.

What changed

• explains why && and || are usually safer in Elixir
• documents when and and or are still appropriate
• adds examples covering structs, nil, and changeset/database-style flows

Why

Elixir’s and and or require a boolean left-hand side, which can raise when used with values like structs or nil. This rule makes that distinction explicit and gives contributors a consistent default.

Notes

• docs-only change
• no runtime or test changes

[Vitaly-A-Ivanov]

Added this to make the and/or vs &&/|| distinction explicit in the Elixir usage rules, with examples for the common nil/struct case. Happy to adjust the wording or trim the examples if you’d prefer it shorter.

[josevalim]

Thank you but our general guidance with the AGENTS.md is that we only add rules that the Phoenix team runs into. This is because if we were to collect everyone's input, even though they are appreciated, they would quickly run out of control.

As most things generated by Phoenix, they are a starting point, so please expand them based on your needs! Thanks!