spec: add subtitles to section on "for" statements

This matches what we already do for switch statements and makes
this large section more visibly organized. No other changes besides
introducing the titles.

Fixes #4486.

Change-Id: I73f274e4fdd27c6cfeaed79090b4553e57a9c479
Reviewed-on: https://go-review.googlesource.com/33410Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

doc/go_spec.html

@@ -4799,8 +4799,8 @@ The "fallthrough" statement is not permitted in a type switch.
 <h3 id="For_statements">For statements</h3>
 
 <p>
-A "for" statement specifies repeated execution of a block. The iteration is
-controlled by a condition, a "for" clause, or a "range" clause.
+A "for" statement specifies repeated execution of a block. There are three forms:
+The iteration may be controlled by a single condition, a "for" clause, or a "range" clause.
 </p>
 
 <pre class="ebnf">
@@ -4808,6 +4808,8 @@ ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
 Condition = Expression .
 </pre>
 
+<h4 id="For_condition">For statements with single condition</h4>
+
 <p>
 In its simplest form, a "for" statement specifies the repeated execution of
 a block as long as a boolean condition evaluates to true.
@@ -4822,6 +4824,8 @@ for a &lt; b {
 }
 </pre>
 
+<h4 id="For_clause">For statements with <code>for</code> clause</h4>
+
 <p>
 A "for" statement with a ForClause is also controlled by its condition, but
 additionally it may specify an <i>init</i>
@@ -4860,6 +4864,8 @@ for cond { S() }    is the same as    for ; cond ; { S() }
 for      { S() }    is the same as    for true     { S() }
 </pre>
 
+<h4 id="For_range">For statements with <code>range</code> clause</h4>
+
 <p>
 A "for" statement with a "range" clause
 iterates through all entries of an array, slice, string or map,