docs: update addressBook get and getMultiaddrsForPeer return values

doc/API.md

@@ -760,7 +760,7 @@ Get the known [`Addresses`][address] of a provided peer.
 
 | Type | Description |
 |------|-------------|
-| `Array<Address>` | Array of peer's [`Addresses`][address] containing the multiaddr and its metadata |
+| `Array<Address>|undefined` | Array of peer's [`Addresses`][address] containing the multiaddr and its metadata if available, otherwise undefined |
 
 #### Example
 
@@ -797,7 +797,7 @@ Get the known `Multiaddr` of a provided peer. All returned multiaddrs will inclu
 
 | Type | Description |
 |------|-------------|
-| `Array<Multiaddr>` | Array of peer's multiaddr |
+| `Array<Multiaddr>|undefined` | Array of peer's multiaddr if available, otherwise undefined |
 
 #### Example
 

src/peer-store/address-book.js

@@ -168,8 +168,9 @@ class AddressBook extends Book {
   /**
    * Get the known multiaddrs for a given peer. All returned multiaddrs
    * will include the encapsulated `PeerId` of the peer.
+   * Returns `undefined` if there are no known multiaddrs for the given peer.
    * @param {PeerId} peerId
-   * @returns {Array<Multiaddr>}
+   * @returns {Array<Multiaddr>|undefined}
    */
   getMultiaddrsForPeer (peerId) {
     if (!PeerId.isPeerId(peerId)) {

src/peer-store/book.js

@@ -77,8 +77,9 @@ class Book {
 
   /**
    * Get the known data of a provided peer.
+   * Returns `undefined` if there is no available data for the given peer.
    * @param {PeerId} peerId
-   * @returns {Array<Data>}
+   * @returns {Array<Data>|undefined}
    */
   get (peerId) {
     if (!PeerId.isPeerId(peerId)) {