spec: better organization of arithmetic operator section

First step towards cleaning up the operator section - no language
changes. Specifically:

- Grouped arithmetic operations by types (integer, floating-point,
  string), with corresponding h4 headings.

- Changed Operator precedence title from h3 to h4.

- Moved Integer Overflow section after integer operations and changed
  its title from h3 to h4.

This puts things that belong together closer. No heading id's were
lost (in case of references from outside the spec).

Change-Id: I6b349ba8d86a6ae29b596beb297cc45c81e69399
Reviewed-on: https://go-review.googlesource.com/13143Reviewed-by: Rob Pike <r@golang.org>

doc/go_spec.html

@@ -3341,7 +3341,8 @@ var v float32 = 1<<s   // illegal: 1 has type float32, cannot shift
 var w int64 = 1.0<<33  // 1.0<<33 is a constant shift expression
 </pre>
 
-<h3 id="Operator_precedence">Operator precedence</h3>
+
+<h4 id="Operator_precedence">Operator precedence</h4>
 <p>
 Unary operators have the highest precedence.
 As the  <code>++</code> and <code>--</code> operators form
@@ -3383,9 +3384,9 @@ x == y+1 &amp;&amp; &lt;-chanPtr &gt; 0
 <p>
 Arithmetic operators apply to numeric values and yield a result of the same
 type as the first operand. The four standard arithmetic operators (<code>+</code>,
-<code>-</code>,  <code>*</code>, <code>/</code>) apply to integer,
-floating-point, and complex types; <code>+</code> also applies
-to strings. All other arithmetic operators apply to integers only.
+<code>-</code>, <code>*</code>, <code>/</code>) apply to integer,
+floating-point, and complex types; <code>+</code> also applies to strings.
+The bitwise logical and shift operators apply to integers only.
 </p>
 
 <pre class="grammar">
@@ -3404,19 +3405,9 @@ to strings. All other arithmetic operators apply to integers only.
 &gt;&gt;   right shift            integer &gt;&gt; unsigned integer
 </pre>
 
-<p>
-Strings can be concatenated using the <code>+</code> operator
-or the <code>+=</code> assignment operator:
-</p>
 
-<pre>
-s := "hi" + string(c)
-s += " and good bye"
-</pre>
+<h4 id="Integer_operators">Integer operators</h4>
 
-<p>
-String addition creates a new string by concatenating the operands.
-</p>
 <p>
 For two integer values <code>x</code> and <code>y</code>, the integer quotient
 <code>q = x / y</code> and remainder <code>r = x % y</code> satisfy the following
@@ -3493,16 +3484,8 @@ follows:
                                       and  m = -1 for signed x
 </pre>
 
-<p>
-For floating-point and complex numbers,
-<code>+x</code> is the same as <code>x</code>,
-while <code>-x</code> is the negation of <code>x</code>.
-The result of a floating-point or complex division by zero is not specified beyond the
-IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a>
-occurs is implementation-specific.
-</p>
 
-<h3 id="Integer_overflow">Integer overflow</h3>
+<h4 id="Integer_overflow">Integer overflow</h4>
 
 <p>
 For unsigned integer values, the operations <code>+</code>,
@@ -3523,6 +3506,35 @@ not occur. For instance, it may not assume that <code>x &lt; x + 1</code> is alw
 </p>
 
 
+<h4 id="Floating_point_operators">Floating-point operators</h4>
+
+<p>
+For floating-point and complex numbers,
+<code>+x</code> is the same as <code>x</code>,
+while <code>-x</code> is the negation of <code>x</code>.
+The result of a floating-point or complex division by zero is not specified beyond the
+IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a>
+occurs is implementation-specific.
+</p>
+
+
+<h4 id="String_concatenation">String concatenation</h4>
+
+<p>
+Strings can be concatenated using the <code>+</code> operator
+or the <code>+=</code> assignment operator:
+</p>
+
+<pre>
+s := "hi" + string(c)
+s += " and good bye"
+</pre>
+
+<p>
+String addition creates a new string by concatenating the operands.
+</p>
+
+
 <h3 id="Comparison_operators">Comparison operators</h3>
 
 <p>