ExternalProject: Clarify `BYPRODUCTS` docs

Clarify that the `BUILD_BYPRODUCTS` and `BYPRODUCTS` options
in ExternalProject may also be required when using the Ninja
generator (as that's one of the primary reasons why you'd want to
use those options).

I've also mentioned that `add_custom_command` has additional
documentation on what `BYPRODUCTS` does.

Modules/ExternalProject.cmake

@@ -637,8 +637,11 @@ External Project Definition
 
       Specifies files that will be generated by the build command but which
       might or might not have their modification time updated by subsequent
-      builds. These ultimately get passed through as ``BYPRODUCTS`` to the
-      build step's own underlying call to :command:`add_custom_command`.
+      builds. This may also be required to explicitly declare dependencies
+      when using the :generator:`Ninja` generator.
+      These ultimately get passed through as ``BYPRODUCTS`` to the
+      build step's own underlying call to :command:`add_custom_command`, which
+      has additional documentation.
 
   **Install Step Options:**
     If the configure step assumed the external project uses CMake as its build
@@ -943,9 +946,12 @@ control needed to implement such step-level capabilities.
     .. versionadded:: 3.2
 
     Files that will be generated by this custom step but which might or might
-    not have their modification time updated by subsequent builds. This list of
+    not have their modification time updated by subsequent builds.
+    This may also be required to explicitly declare dependencies
+    when using the :generator:`Ninja` generator. This list of
     files will ultimately be passed through as the ``BYPRODUCTS`` option to the
-    :command:`add_custom_command` used to implement the custom step internally.
+    :command:`add_custom_command` used to implement the custom step internally,
+    which has additional documentation.
 
   ``ALWAYS <bool>``
     When enabled, this option specifies that the custom step should always be