Update MuxedAddress handling and transfer guidelines

[notarobot44]

Clarified handling of MuxedAddress in transfer operations and emphasized the importance of passing &Address directly to avoid compilation issues.

[stellar-triage-bot]

🤖 Automated message from the Stellar Triage Bot.

Thanks for this. It is a useful pitfall to name, and it saves readers a confusing compile error.

I checked it against the soroban-sdk source for v27 (the version the skill pins) and v28:

• The generated client types the to argument as impl Into<MuxedAddress> (fn_arg_make_into in soroban-sdk-macros).
• MuxedAddress has both From<&Address> and From<&MuxedAddress>. So &to.into() has two valid targets and fails with E0283.
• Passing &to works through From<&Address>. All other transfer calls in skills/ already do this.

No other file in skills/ repeats the old wording. Merging now.

[stellar-triage-bot]

Verified against soroban-sdk v27 and v28 source.