spec: define that initialization is sequential

This is true of the existing implementations, and I think
it is an important property to guarantee.

R=golang-dev, r, borman, r
CC=golang-dev
https://golang.org/cl/5321058

doc/go_spec.html

@@ -5149,12 +5149,22 @@ func main() { … }
 <p>
 Program execution begins by initializing the main package and then
 invoking the function <code>main</code>.
-</p>
-<p>
 When the function <code>main</code> returns, the program exits.
 It does not wait for other (non-<code>main</code>) goroutines to complete.
 </p>
 
+<p>
+Package initialization&mdash;variable initialization and the invocation of 
+<code>init</code> functions&mdash;happens in a single goroutine,
+sequentially, one package at a time.
+An <code>init</code> function may launch other goroutines, which can run
+concurrently with the initialization code. However, initialization
+always sequences
+the <code>init</code> functions: it will not start the next
+<code>init</code> until
+the previous one has returned.
+</p>
+
 <h2 id="Run_time_panics">Run-time panics</h2>
 
 <p>