Fix confusing description of JsonPrimitive.content (Kotlin#2670)

- clarifies that JsonPrimitive.content will return `"null"` rather than `null`
- include the suggestion that `contentOrNull` should be used if the JsonPrimitive could be `JsonNull`
- fix potentially misleading usage in `JsonTreeTest`

Author: gyuhyeon

formats/json-tests/commonTest/src/kotlinx/serialization/json/serializers/JsonTreeTest.kt

@@ -47,7 +47,7 @@ class JsonTreeTest : JsonTestBase() {
         assertTrue(elem.getValue("c") is JsonArray)
 
         val array = elem.getValue("c").jsonArray
-        assertEquals("foo", array.getOrNull(0)?.jsonPrimitive?.content)
+        assertEquals("foo", array.getOrNull(0)?.jsonPrimitive?.contentOrNull)
         assertEquals(100500, array.getOrNull(1)?.jsonPrimitive?.int)
 
         assertTrue(array[2] is JsonObject)

formats/json/commonMain/src/kotlinx/serialization/json/JsonElement.kt

@@ -41,7 +41,8 @@ public sealed class JsonPrimitive : JsonElement() {
     public abstract val isString: Boolean
 
     /**
-     * Content of given element without quotes. For [JsonNull] this methods returns `null`
+     * Content of given element without quotes. For [JsonNull], this method returns a "null" string.
+     * [JsonPrimitive.contentOrNull] should be used for [JsonNull] to get a `null`.
      */
     public abstract val content: String