Expand `OverflowBehavior` and `OverfundingPolicy` docs to explain their relationship

[Kingsman-99]

Description

Both OverflowBehavior and OverfundingPolicy appear in contracts/split/src/types.rs and govern overpayment handling. Their relationship is subtle (OverfundingPolicy::Cap delegates to OverflowBehavior). The existing comments are brief; expanding them with a # Relationship section explaining which takes precedence would prevent misuse.

Acceptance Criteria

• Add an expanded doc comment to both OverflowBehavior and OverfundingPolicy in contracts/split/src/types.rs
• The comment for OverfundingPolicy::Cap explains it delegates to OverflowBehavior
• The comment for OverfundingPolicy includes a # Relationship section referencing OverflowBehavior
• cargo doc builds without warnings
• cargo test passes

Context

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