Point to exec_start from start in init documentation

Since it mentions a caveat, it's nice to know how to avoid
that caveat.

Test: N/A
Change-Id: I487f1080058bf83a76a725735c6f283f88b0739e
diff --git a/init/README.md b/init/README.md
index e8df4ec..b8300fa 100644
--- a/init/README.md
+++ b/init/README.md
@@ -583,6 +583,7 @@
   Note that this is _not_ synchronous, and even if it were, there is
   no guarantee that the operating system's scheduler will execute the
   service sufficiently to guarantee anything about the service's status.
+  See the `exec_start` command for a synchronous version of `start`.
 
 > This creates an important consequence that if the service offers
   functionality to other services, such as providing a