spec: specify order of init() calls

The spec did not specify the order in which
init() functions are called. Specify that
they are called in source order since we have
now also specified the initialization order
of independent variables.

While technically a language change, no
existing code could have relied on this,
so this should not break anything.

Per suggestion from rsc.

LGTM=r, iant
R=rsc, iant, r, ken
CC=golang-codereviews
https://golang.org/cl/98420046

doc/go_spec.html

@@ -6008,7 +6008,8 @@ in a program.
 <p>
 A package with no imports is initialized by assigning initial values
 to all its package-level variables followed by calling all <code>init</code>
-functions in unspecified order.
+functions in the order they appear in the source, possibly in multiple files,
+as presented to the compiler.
 If a package has imports, the imported packages are initialized
 before initializing the package itself. If multiple packages import
 a package, the imported package will be initialized only once.