add docs and fix snippet name

Author: dlarocque

google-cloud-firestore/src/main/java/com/google/cloud/firestore/pipeline/expressions/Expression.java

@@ -3130,12 +3130,26 @@ public static Expression type(String fieldName) {
     return type(field(fieldName));
   }
 
+  /**
+   * Creates an expression that checks if the result of this expression is of the given type.
+   *
+   * @param expr The expression to check the type of.
+   * @param type The type to check for.
+   * @return A new {@link BooleanExpression} that evaluates to true if the expression's result is of the given type, false otherwise.
+   */
   @BetaApi
   public static BooleanExpression isType(Expression expr, Type type) {
     return new BooleanFunctionExpression(
         "is_type", ImmutableList.of(expr, constant(type.name().toLowerCase())));
   }
 
+  /**
+   * Creates an expression that checks if the result of this expression is of the given type.
+   *
+   * @param fieldName The name of the field to check the type of.
+   * @param type The type to check for.
+   * @return A new {@link BooleanExpression} that evaluates to true if the expression's result is of the given type, false otherwise.
+   */
   @BetaApi
   public static BooleanExpression isType(String fieldName, Type type) {
     return new BooleanFunctionExpression(
@@ -4809,6 +4823,12 @@ public final Expression type() {
     return type(this);
   }
 
+  /**
+   * Creates an expression that checks if the result of this expression is of the given type.
+   *
+   * @param type The type to check for.
+   * @return A new {@link BooleanExpression} that evaluates to true if the expression's result is of the given type, false otherwise.
+   */
   @BetaApi
   public final Expression isType(Type type) {
     return isType(this, type);

samples/preview-snippets/src/main/java/com/example/firestore/PipelineSnippets.java

@@ -1553,15 +1553,15 @@ void vectorLengthFunction() throws ExecutionException, InterruptedException {
   }
 
   void isTypeFunction() throws ExecutionException, InterruptedException {
-    // [START vector_length]
+    // [START is_type]
     Pipeline.Snapshot result =
         firestore
             .pipeline()
             .collection("books")
             .select(field("rating").isType(Type.INT64).as("isRatingInt64"))
             .execute()
             .get();
-    // [END vector_length]
+    // [END is_type]
     System.out.println(result.getResults());
   }