docs: document sqlx test connection limits

[angelnereira]

Summary

• document how #[sqlx::test] interacts with database connection limits when many tests run concurrently
• show how to use the PoolOptions test signature to reduce the per-test pool size
• mention cargo test -- --test-threads <N> as a way to limit Rust test runner concurrency

This is related to #2567.

Checks

• cargo fmt
• cargo test -p sqlx --doc --no-default-features --features postgres,macros,migrate,runtime-tokio
• cargo check -p sqlx --no-default-features --features postgres,macros,migrate,runtime-tokio
• git diff --check