Improve javadoc regarding use on @Bean methods

Closes gh-32166

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/condition/ConditionalOnClass.java

@@ -28,11 +28,33 @@ import org.springframework.context.annotation.Conditional;
  * {@link Conditional @Conditional} that only matches when the specified classes are on
  * the classpath.
  * <p>
- * A {@link #value()} can be safely specified on {@code @Configuration} classes as the
+ * A {@code Class} {@link #value() value} can be safely specified on
- * annotation metadata is parsed by using ASM before the class is loaded. Extra care is
+ * {@code @Configuration} classes as the annotation metadata is parsed by using ASM before
- * required when placed on {@code @Bean} methods, consider isolating the condition in a
+ * the class is loaded. If a class reference cannot be used then a {@link #name() name}
- * separate {@code Configuration} class, in particular if the return type of the method
+ * {@code String} attribute can be used.
- * matches the {@link #value target of the condition}.
+ * <p>
+ * <b>Note:</b> Extra care must be taken when using {@code @ConditionalOnClass} on
+ * {@code @Bean} methods where typically the return type is the target of the condition.
+ * Before the condition on the method applies, the JVM will have loaded the class and
+ * potentially processed method references which will fail if the class is not present. To
+ * handle this scenario, a separate {@code @Configuration} class should be used to isolate
+ * the condition. For example: <pre class="code">
+ * &#064;Configuration(proxyBeanMethods = false)
+ * public class MyAutoConfiguration {
+ *
+ * 	&#64;Configuration(proxyBeanMethods = false)
+ * 	&#64;ConditionalOnClass(SomeService.class)
+ * 	public static class SomeServiceConfiguration {
+ *
+ * 		&#64;Bean
+ * 		&#64;ConditionalOnMissingBean
+ * 		public SomeService someService() {
+ * 			return new SomeService();
+ * 		}
+ *
+ * 	}
+ *
+ * }</pre>
  *
  * @author Phillip Webb
  * @since 1.0.0