Document #[contractimpl] export name behaviour

[leighmcculloch]

What

Add a Notes section to the #[contractimpl] macro documentation explaining that export names are derived from function names alone (no type prefix), that function names must be unique across all #[contractimpl] blocks in a crate to avoid Wasm export collisions, and that importing a crate containing #[contractimpl] blocks pulls its exported functions into the importing binary.

Why

Both behaviors — duplicate names across impl blocks and transitive re-export from dependencies — are non-obvious and were not documented on the contractimpl macro.

Close #1808

[Copilot]

Pull request overview

Adds clarifying documentation to #[contractimpl] explaining how Wasm export names are derived and why that can lead to surprising export collisions, including via dependencies.

Changes:

• Document that Wasm export names are derived from the method name only (no type prefix/namespace).
• Document the requirement that method names be unique across all #[contractimpl] blocks within a crate to avoid export collisions.
• Document that linking a dependency that contains #[contractimpl] can bring its exports into the final Wasm binary, potentially causing unexpected exports/collisions.