fix: Improve javadocs for AppShellConfigurator (#10775)

Currently it scares non-frontend developers who scare to touch anything that says .html

Co-authored-by: Pekka Hyvönen <pekka@vaadin.com>
Co-authored-by: Mikhail Shabarov <61410877+mshabarov@users.noreply.github.com>

flow-server/src/main/java/com/vaadin/flow/component/page/AppShellConfigurator.java

@@ -22,9 +22,12 @@
 import com.vaadin.flow.server.AppShellSettings;
 
 /**
- * A marker interface to configure the index.hml page when using client-side
- * bootstrapping. This class supports the following annotations that affect the
- * generated index.html page (also known as 'application shell'):
+ * An interface to configure application features and the host page where the 
+ * Vaadin application is running. 
+ * It automatically configures the index.html page. 
+ * Configuration can be done using a class implementing this interface with 
+ * following annotations that affect the generated index.html page (also known
+ * as 'application shell'):
  *
  * <ul>
  * <li>{@link Meta}: appends an HTML {@code <meta>} tag to the bottom of the
@@ -40,8 +43,8 @@
  *
  * <p>
  * There is a single application shell for the entire Vaadin application, and
- * therefore there can be at max one class implementing
- * {@link AppShellConfigurator}.
+ * there can only be one class implementing
+ * {@link AppShellConfigurator} per Application.
  * </p>
  *
  * <p>