Add `///` doc comment to `StorageKey` enum explaining the 50-variant Soroban limit

[Kingsman-99]

Description

contracts/split/src/storage_keys.rs has a module-level comment explaining the four-enum split, but StorageKey itself lacks a /// doc comment. New contributors editing the enum may not understand why it can have at most 50 variants. A /// doc comment on the enum itself reinforces this constraint.

Acceptance Criteria

• Add a /// doc comment to pub enum StorageKey explaining the 50-variant Soroban limit and that new keys should be appended (never reordered)
• cargo doc builds without warnings
• cargo test passes

Context

• Target file: contracts/split/src/storage_keys.rs