context: Add docs describing common Key usage

This recently came up in https://stackoverflow.com/a/67062503/4690866,
but it has come up multiple times before. These docs aren't ideal, as
they may be missed by a reader and so references in other parts of the
API would probably be appropriate. There could also be something about
"Context is not a general purpose map." But this is an improvement, and
I didn't want to let the perfect be the enemy of the good.

context/src/main/java/io/grpc/Context.java

@@ -955,7 +955,12 @@ public class Context {
   }
 
   /**
-   * Key for indexing values stored in a context.
+   * Key for indexing values stored in a context. Keys use reference equality and Context does not
+   * provide a mechanism to loop over Keys. This means there is no way to access a Key's value from
+   * a Context without having access to the Key instance itself. This allows strong control over
+   * what code can get/set a key in the Context. For example, you might manage access to Key similar
+   * to a ThreadLocal using Java visibility (private/protected). Generally Keys are stored in static
+   * fields.
    */
   public static final class Key<T> {
     private final String name;