Clarify the behavior of getAll if additional keys are loaded

This is already documented on the cache loaders for their bulk
loading methods:
```
If the returned map contains extra keys not present in {@code keys}
then all returned entries will be cached, but only the entries for
{@code keys} will be returned from {@code getAll}.
```

caffeine/src/main/java/com/github/benmanes/caffeine/cache/AsyncCache.java

@@ -108,8 +108,9 @@ CompletableFuture<V> get(@NonNull K key,
    * <p>
    * A single request to the {@code mappingFunction} is performed for all keys which are not already
    * present in the cache. If another call to {@link #get} tries to load the value for a key in
-   * {@code keys}, that thread retrieves a future that is completed by this bulk computation. Note
-   * that multiple threads can concurrently load values for distinct keys.
+   * {@code keys}, that thread retrieves a future that is completed by this bulk computation. Any
+   * loaded values for keys that were not specifically requested will not be returned, but will be
+   * stored in the cache. Note that multiple threads can concurrently load values for distinct keys.
    * <p>
    * Note that duplicate elements in {@code keys}, as determined by {@link Object#equals}, will be
    * ignored.
@@ -138,8 +139,9 @@ default CompletableFuture<Map<K, V>> getAll(@NonNull Iterable<? extends @NonNull
    * <p>
    * A single request to the {@code mappingFunction} is performed for all keys which are not already
    * present in the cache. If another call to {@link #get} tries to load the value for a key in
-   * {@code keys}, that thread retrieves a future that is completed by this bulk computation. Note
-   * that multiple threads can concurrently load values for distinct keys.
+   * {@code keys}, that thread retrieves a future that is completed by this bulk computation. Any
+   * loaded values for keys that were not specifically requested will not be returned, but will be
+   * stored in the cache. Note that multiple threads can concurrently load values for distinct keys.
    * <p>
    * Note that duplicate elements in {@code keys}, as determined by {@link Object#equals}, will be
    * ignored.

caffeine/src/main/java/com/github/benmanes/caffeine/cache/Cache.java

@@ -105,7 +105,9 @@ public interface Cache<K, V> {
    * the value for a key in {@code keys}, implementations may either have that thread load the entry
    * or simply wait for this thread to finish and return the loaded value. In the case of
    * overlapping non-blocking loads, the last load to complete will replace the existing entry. Note
-   * that multiple threads can concurrently load values for distinct keys.
+   * that multiple threads can concurrently load values for distinct keys. Any loaded values for
+   * keys that were not specifically requested will not be returned, but will be stored in the
+   * cache.
    * <p>
    * Note that duplicate elements in {@code keys}, as determined by {@link Object#equals}, will be
    * ignored.