docs: wallet addresses and payment pointers

[hajjimo]

Creating this issue based on feedback received from Don of Fynbos.

While the glossary has an entry for payment pointers that describes them as a type of wallet address, they are only mentioned on the Interledger page, and their relationship with wallet addresses is never mentioned on the wallet addresses page. We should note that wallet addresses are universal account identifiers, and payment pointers are specific to ILP on the wallet addresses page.