Improve Kotlin extensions doc about type erasure

Since type erasure can be fixed only when using
ParameterizedTypeReference based Java methods, TestRestTemplate
API documentation should be updated to specify which extensions
are subject to type erasure, and which are not.

Closes gh-11604

spring-boot-project/spring-boot-test/src/test/kotlin/org/springframework/boot/test/web/client/TestRestTemplateExtensionsTests.kt

@@ -122,6 +122,13 @@ class TestRestTemplateExtensionsTests {
 		verify(template, times(1)).patchForObject(url, body, Foo::class.java)
 	}
 
+	@Test
+	fun `patchForObject with reified type parameters`() {
+		val url = "https://spring.io"
+		template.patchForObject<Foo>(url)
+		verify(template, times(1)).patchForObject(url, null, Foo::class.java)
+	}
+
 	@Test
 	fun `postForObject with reified type parameters, String, Any and varargs`() {
 		val url = "https://spring.io"
@@ -149,6 +156,13 @@ class TestRestTemplateExtensionsTests {
 		verify(template, times(1)).postForObject(url, body, Foo::class.java)
 	}
 
+	@Test
+	fun `postForObject with reified type parameters`() {
+		val url = "https://spring.io"
+		template.postForObject<Foo>(url)
+		verify(template, times(1)).postForObject(url, null, Foo::class.java)
+	}
+
 	@Test
 	fun `postForEntity with reified type parameters, String, Any and varargs`() {
 		val url = "https://spring.io"
@@ -176,6 +190,13 @@ class TestRestTemplateExtensionsTests {
 		verify(template, times(1)).postForEntity(url, body, Foo::class.java)
 	}
 
+	@Test
+	fun `postForEntity with reified type parameters`() {
+		val url = "https://spring.io"
+		template.postForEntity<Foo>(url)
+		verify(template, times(1)).postForEntity(url, null, Foo::class.java)
+	}
+
 	@Test
 	fun `exchange with reified type parameters, String, HttpMethod, HttpEntity and varargs`() {
 		val url = "https://spring.io"
@@ -217,6 +238,15 @@ class TestRestTemplateExtensionsTests {
 				object : ParameterizedTypeReference<List<Foo>>() {})
 	}
 
+	@Test
+	fun `exchange with reified type parameters, String and HttpMethod`() {
+		val url = "https://spring.io"
+		val method = HttpMethod.GET
+		template.exchange<List<Foo>>(url, method)
+		verify(template, times(1)).exchange(url, method, null,
+				object : ParameterizedTypeReference<List<Foo>>() {})
+	}
+
 	@Test
 	fun `RestOperations are available`() {
 		val extensions = Class.forName(