Add example with set extension methods to memory cache usage (#20850)

* Add examples with Set extension methods

* Just one example

* Add more description.

aspnetcore/performance/caching/memory.md

@@ -67,6 +67,9 @@ The current time and the cached time are displayed:
 
 [!code-cshtml[](memory/3.0sample/WebCacheSample/Views/Home/Cache.cshtml)]
 
+The following code uses the [Set](/dotnet/api/microsoft.extensions.caching.memory.cacheextensions.set#Microsoft_Extensions_Caching_Memory_CacheExtensions_Set__1_Microsoft_Extensions_Caching_Memory_IMemoryCache_System_Object___0_System_TimeSpan_) extension method to cache data for a relative time without creating the `MemoryCacheEntryOptions` object.
+[!code-csharp[](memory/3.0sample/WebCacheSample/Controllers/HomeController.cs?name=snippet_set)]
+
 The cached `DateTime` value remains in the cache while there are requests within the timeout period.
 
 The following code uses [GetOrCreate](/dotnet/api/microsoft.extensions.caching.memory.cacheextensions.getorcreate#Microsoft_Extensions_Caching_Memory_CacheExtensions_GetOrCreate__1_Microsoft_Extensions_Caching_Memory_IMemoryCache_System_Object_System_Func_Microsoft_Extensions_Caching_Memory_ICacheEntry___0__) and [GetOrCreateAsync](/dotnet/api/microsoft.extensions.caching.memory.cacheextensions.getorcreateasync#Microsoft_Extensions_Caching_Memory_CacheExtensions_GetOrCreateAsync__1_Microsoft_Extensions_Caching_Memory_IMemoryCache_System_Object_System_Func_Microsoft_Extensions_Caching_Memory_ICacheEntry_System_Threading_Tasks_Task___0___) to cache data.

aspnetcore/performance/caching/memory/3.0sample/WebCacheSample/Controllers/HomeController.cs

@@ -79,6 +79,25 @@ public class HomeController : Controller
     }
     #endregion
 
+    #region snippet_set
+    public IActionResult SetCacheRelativeExpiration()
+    {
+        DateTime cacheEntry;
+
+        // Look for cache key.
+        if (!_cache.TryGetValue(CacheKeys.Entry, out cacheEntry))
+        {
+            // Key not in cache, so get data.
+            cacheEntry = DateTime.Now;
+
+            // Save data in cache and set the relative expiration time to one day
+            _cache.Set(CacheKeys.Entry, cacheEntry, TimeSpan.FromDays(1));
+        }
+
+        return View("Cache", cacheEntry);
+    }
+    #endregion
+
     public IActionResult CacheRemove()
     {
         _cache.Remove(CacheKeys.Entry);