doc: update `Select` function description

Capture potential performance slow down for `Select` by network & clarify
return values.

src/addrman.h

@@ -148,8 +148,10 @@ public:
      *
      * @param[in] new_only Whether to only select addresses from the new table. Passing `true` returns
      *                     an address from the new table or an empty pair. Passing `false` will return an
-     *                     address from either the new or tried table (it does not guarantee a tried entry).
-     * @param[in] network  Select only addresses of this network (nullopt = all)
+     *                     empty pair or an address from either the new or tried table (it does not
+     *                     guarantee a tried entry).
+     * @param[in] network  Select only addresses of this network (nullopt = all). Passing a network may
+     *                     slow down the search.
      * @return    CAddress The record for the selected peer.
      *            seconds  The last time we attempted to connect to that peer.
      */