Make doc clearer on getCrossSigningKeyId (#4477)

* Make doc clearer on getCrossSigningKeyId

I was trying to work out why this was being used in a check. It
turns out it only returns the key ID if the private part is stored
locally, which seems very much non-obvious.

* Better doc

* Formatting & clarity

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

---------

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

src/crypto-api/index.ts

@@ -274,7 +274,13 @@ export interface CryptoApi {
     isCrossSigningReady(): Promise<boolean>;
 
     /**
-     * Get the ID of one of the user's cross-signing keys.
+     * Get the ID of one of the user's cross-signing keys, if both private and matching
+     * public parts of that key are available (ie. cached in the local crypto store).
+     *
+     * The public part may not be available if a `/keys/query` request has not yet been
+     * performed, or if the device that created the keys failed to publish them.
+     *
+     * If either part of the keypair is not available, this will return `null`.
      *
      * @param type - The type of key to get the ID of.  One of `CrossSigningKey.Master`, `CrossSigningKey.SelfSigning`,
      *     or `CrossSigningKey.UserSigning`.  Defaults to `CrossSigningKey.Master`.