docs(macros): document that bind parameter nullability is not compile-checked

[jeremie0342]

What

Restructures the ## Nullability: Bind Parameters section in the query! macro documentation (src/macros/mod.rs).

Why

Users regularly report the same confusion (e.g. #2642): they write sqlx::query!("INSERT INTO foo (data) VALUES (\$1)", None::<String>) against a NOT NULL column, expecting a compile-time error, and are surprised when the code compiles but fails at runtime.

The existing section technically covered this (Option::None will be bound as NULL, so ... be sure your query can support it), but the phrasing understated the limitation and hid it behind guidance about WHERE clauses.

What changed

• New opening paragraph states upfront, in bold, that bind parameter nullability is not verified at compile time.
• Adds a short explanation of why (SQLx does not parse SQL) with a link to the FAQ entry.
• Adds the concrete failing example from Query macro documentation understates lack of nullability checking on bind parameters #2642 as a runnable-looking snippet.
• Provides two actionable workarounds (stricter Rust types, integration tests).
• The pre-existing guidance about WHERE clause / NULL comparisons is preserved as a sub-section (### Bind parameters in WHERE clauses).

Scope

• One file changed: src/macros/mod.rs
• Documentation only, no code change, no behavior change
• cargo doc -p sqlx --no-deps produces no new warnings in the edited range

Closes #2642