Update docs for some important annotations.

Author: greenrobot-team

objectbox-java-api/src/main/java/io/objectbox/annotation/Id.java

@@ -22,7 +22,11 @@
 import java.lang.annotation.Target;
 
 /**
- * Marks field is the primary key of the entity's table
+ * Marks the ID property of an {@link Entity @Entity}.
+ * The property must be of type long (or Long in Kotlin) and have not-private visibility
+ * (or a not-private getter and setter method).
+ * <p>
+ * ID properties are unique and indexed by default.
  */
 @Retention(RetentionPolicy.CLASS)
 @Target(ElementType.FIELD)
@@ -35,8 +39,10 @@
 //    boolean monotonic() default false;
 
     /**
-     * Allows IDs to be assigned by the developer. This may make sense for using IDs originating somewhere else, e.g.
-     * from the server.
+     * Allows IDs of new entities to be assigned manually.
+     * Warning: This has side effects, check the online documentation on self-assigned object IDs for details.
+     * <p>
+     * This may allow re-use of IDs assigned elsewhere, e.g. by a server.
      */
     boolean assignable() default false;
 }

objectbox-java-api/src/main/java/io/objectbox/annotation/Index.java

@@ -21,15 +21,20 @@
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
-import io.objectbox.annotation.apihint.Internal;
-
 /**
- * Specifies that the property should be indexed, which is highly recommended if you do queries using this property.
- *
- * To fine tune indexing you can specify {@link IndexType} if necessary.
+ * Specifies that the property should be indexed.
+ * <p>
+ * It is highly recommended to index properties that are used in a query to improve query performance.
+ * <p>
+ * To fine tune indexing of a property you can override the default index {@link #type()}.
+ * <p>
+ * Note: indexes are currently not supported for byte array, float or double properties.
  */
 @Retention(RetentionPolicy.CLASS)
 @Target(ElementType.FIELD)
 public @interface Index {
+    /**
+     * Sets the {@link IndexType}, defaults to {@link IndexType#DEFAULT}.
+     */
     IndexType type() default IndexType.DEFAULT;
 }

objectbox-java-api/src/main/java/io/objectbox/annotation/IndexType.java

@@ -17,20 +17,22 @@
 package io.objectbox.annotation;
 
 /**
- * ObjectBox offers three index types, from which it chooses from with a reasonable default (see {@link #DEFAULT}).
+ * ObjectBox offers a value and two hash index types, from which it chooses a reasonable default (see {@link #DEFAULT}).
  * <p>
  * For some queries/use cases it might make sense to override the default choice for optimization purposes.
+ * <p>
+ * Note: hash indexes are currently only supported for string properties.
  */
 public enum IndexType {
     /**
      * Use the default index type depending on the property type:
-     * {@link #VALUE} for scalars and {@link #HASH} for Strings and byte arrays.
+     * {@link #VALUE} for scalars and {@link #HASH} for Strings.
      */
     DEFAULT,
 
     /**
      * Use the property value to build the index.
-     * For Strings and byte arrays this may occupy more space than the default setting.
+     * For Strings this may occupy more space than the default setting.
      */
     VALUE,
 

objectbox-java-api/src/main/java/io/objectbox/annotation/Unique.java

@@ -22,12 +22,12 @@
 import java.lang.annotation.Target;
 
 /**
- * Marks values of a property to be unique.
- * The property will be indexed behind the scenes, just like using @{@link Index}.
- * Thus you do not need to put an extra @{@link Index} on the property, unless you want to configure the index with
- * additional parameters.
- *
- * Trying to put object with offending values will result in a UniqueViolationException.
+ * Enforces that the value of a property is unique among all objects in a box before an object can be put.
+ * <p>
+ * Trying to put an object with offending values will result in a UniqueViolationException.
+ * <p>
+ * Unique properties are based on an {@link Index @Index}, so the same restrictions apply.
+ * It is supported to explicitly add the {@link Index @Index} annotation to configure the index.
  */
 @Retention(RetentionPolicy.CLASS)
 @Target(ElementType.FIELD)