[api] Add better documentation to Isolate::Allocate

R=ulan@chromium.org
CC=​jbroman@chromium.org

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I606a182774188b953247b62e5426ee7feadd1a74
Reviewed-on: https://chromium-review.googlesource.com/1047206
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53105}

include/v8.h

@@ -7257,7 +7257,11 @@ class V8_EXPORT Isolate {
 
   /**
    * Allocates a new isolate but does not initialize it. Does not change the
-   * currently entered isolate. Initialize the isolate by calling
+   * currently entered isolate.
+   *
+   * Only Isolate::GetData() and Isolate::SetData(), which access the
+   * embedder-controlled parts of the isolate, are allowed to be called on the
+   * uninitialized isolate. To initialize the isolate, call
    * Isolate::Initialize().
    *
    * When an isolate is no longer used its resources should be freed