Add docs to Slog class.

Docs will be super useful, especially to new developers.
I see several docs and questions inside/outside google asking
what is it and when to use it.

Bug: 182476140
Test: m -j
Change-Id: Ibb41ab0b6fd808f2a1e5475103860fc415a502d6

core/java/android/util/Slog.java

@@ -26,6 +26,10 @@ import java.util.Formatter;
 import java.util.Locale;
 
 /**
+ * API for sending log output to the {@link Log#LOG_ID_SYSTEM} buffer.
+ *
+ * <p>Should be used by system components. Use {@code adb logcat --buffer=system} to fetch the logs.
+ *
  * @hide
  */
 public final class Slog {