doc: updates notes about super builds (#6448)

Author: coryan

CHANGELOG.md

@@ -4,6 +4,17 @@
 
 <!-- Keep these sorted by estimated date -->
 
+<details>
+<summary>2021-05-01: removing super build test files.</summary>
+
+* Shortly after 2021-05-01 we are planning to remove the super build test files
+  in the `super/` directory. `google-cloud-cpp` should remain usable in a
+  super build for a larger project, but we think this testing is redundant
+  with other testing that installs `google-cloud-cpp` in `$HOME`. Contributors
+  to `google-cloud-cpp` can use `vcpkg` to install the dependencies as part of
+  their CMake setup.
+</details>
+
 <details>
 <summary>2021-06-01: retiring experimental Bigtable Admin Async APIs</summary>
 

super/ExternalProjectHelper.cmake

@@ -20,10 +20,11 @@ include(GNUInstallDirs)
 message(
     WARNING
         [==[
-  Super builds are no longer recommended, and will be removed on 2022-05-01
-  or shortly after.  If you want to automatically download and build the
-  `google-cloud-cpp` dependencies we recommend that you use `vcpkg` as
-  described in /doc/contributor/howto-guide-setup-cmake-environment.md
+  Super builds are no longer recommended, and will be removed shortly
+  after 2021-05-01.  If you want to automatically download and build
+  the `google-cloud-cpp` dependencies we recommend that you use
+  `vcpkg` as described in
+  /doc/contributor/howto-guide-setup-cmake-environment.md
 ]==])
 
 set(GOOGLE_CLOUD_CPP_EXTERNAL_PREFIX