Fix return type of Builder.set (#5161)

* Fix return type of Builder.set

* Add missing comments

* Address feedback

Author: wu-hui

firebase-firestore/api.txt

@@ -345,7 +345,7 @@ package com.google.firebase.firestore {
 
   public static class MemoryLruGcSettings.Builder {
     method @NonNull public com.google.firebase.firestore.MemoryLruGcSettings build();
-    method public void setSizeBytes(long);
+    method @NonNull public com.google.firebase.firestore.MemoryLruGcSettings.Builder setSizeBytes(long);
   }
 
   public enum MetadataChanges {

firebase-firestore/src/main/java/com/google/firebase/firestore/MemoryEagerGcSettings.java

@@ -17,6 +17,17 @@
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 
+/**
+ * Configures the SDK to use an eager garbage collector for memory cache. The eager garbage
+ * collector will attempt to remove any documents from SDK's memory cache as soon as it is no longer
+ * used.
+ *
+ * <p>This is the default garbage collector unless specified explicitly otherwise.
+ *
+ * <p>To use, create an instance using {@code MemoryEagerGcSettings#newBuilder().build()}, then set
+ * the instance to {@code MemoryCacheSettings.Builder#setGcSettings}, and use the built {@code
+ * MemoryCacheSettings} instance to configure the Firestore SDK.
+ */
 public final class MemoryEagerGcSettings implements MemoryGarbageCollectorSettings {
   private MemoryEagerGcSettings() {}
 

firebase-firestore/src/main/java/com/google/firebase/firestore/MemoryLruGcSettings.java

@@ -16,6 +16,13 @@
 
 import androidx.annotation.NonNull;
 
+/**
+ * Configures the SDK to use a Least-Recently-Used garbage collector for memory cache.
+ *
+ * <p>To use, create an instance using {@code MemoryLruGcSettings#newBuilder().build()}, then set
+ * the instance to {@code MemoryCacheSettings.Builder#setGcSettings}, and use the built {@code
+ * MemoryCacheSettings} instance to configure the Firestore SDK.
+ */
 public final class MemoryLruGcSettings implements MemoryGarbageCollectorSettings {
 
   private long sizeBytes;
@@ -30,20 +37,43 @@ public MemoryLruGcSettings build() {
       return new MemoryLruGcSettings(sizeBytes);
     }
 
-    public void setSizeBytes(long size) {
+    /**
+     * Sets an approximate cache size threshold for the memory cache. If the cache grows beyond this
+     * size, Firestore SDK will start removing data that hasn't been recently used. The size is not
+     * a guarantee that the cache will stay below that size, only that if the cache exceeds the
+     * given size, cleanup will be attempted.
+     *
+     * <p>A default size of 100MB (100 * 1024 * 1024) is used if unset. The minimum value to set is
+     * 1 MB (1024 * 1024).
+     *
+     * @return this {@code Builder} instance.
+     */
+    @NonNull
+    public Builder setSizeBytes(long size) {
       sizeBytes = size;
+      return this;
     }
   }
 
   private MemoryLruGcSettings(long size) {
     sizeBytes = size;
   }
 
+  /** Returns a new instance of {@link MemoryLruGcSettings.Builder} with default configurations. */
   @NonNull
   public static MemoryLruGcSettings.Builder newBuilder() {
     return new Builder();
   }
 
+  /**
+   * Returns cache size threshold for the memory cache. If the cache grows beyond this size,
+   * Firestore SDK will start removing data that hasn't been recently used. The size is not a
+   * guarantee that the cache will stay below that size, only that if the cache exceeds the given
+   * size, cleanup will be attempted.
+   *
+   * <p>By default, memory LRU cache is enabled with a cache size of 100MB (100 * 1024 * 1024). The
+   * minimum value is 1 MB (1024 * 1024).
+   */
   public long getSizeBytes() {
     return sizeBytes;
   }