Use proper let expressions in operators table

The spec used to define some operators via pseudocode let expressions
like this:

    let x=y in e

which, with the introduction of actual let expressions into the
language (#3853), can instead be written as:

    let (x=y) e

rendering the previous syntax not only obsolete, but also confusing; see
for example @PhiLhoSoft on the ceylon-users mailing list:
https://groups.google.com/d/msg/ceylon-users/N3idC2CZQxI/N_-BaNdydzgJ

typechecker/en/modules/expressions.xml

@@ -2044,23 +2044,12 @@ Digit{1,2} ":" Digit{2} ( ":" Digit{2} ( ":" Digit{3} )? )?
  <para>In the tables, the following pseudo-code is used, which is not legal
  Ceylon syntax:</para>
 
- <para>First,</para>
-
  <programlisting>if (b) then x else y //pseudocode</programlisting>
 
  <para>means the value of <literal>result</literal> after execution of the
  following:</para>
 
  <programlisting>X result; if (b) { result=x; } else { result=y; }</programlisting>
-
- <para>Second,</para>
-
- <programlisting>let t=x in y //pseudocode</programlisting>
-
- <para>means the value of <literal>result</literal> after execution of the
- following:</para>
-
- <programlisting>X t = x; Y result=y;</programlisting>
 
  </section>
 
@@ -2243,7 +2232,7 @@ Digit{1,2} ":" Digit{2} ( ":" Digit{2} ( ":" Digit{3} )? )?
  <row>
  <entry><literal>lhs in rhs</literal></entry>
  <entry>in</entry>
- <entry><literal>let x=lhs in rhs.contains(x)</literal></entry>
+ <entry><literal>let (x=lhs) rhs.contains(x)</literal></entry>
  <entry><literal>Object</literal></entry>
  <entry><literal>Category</literal></entry>
  <entry><literal>Boolean</literal></entry>
@@ -2298,19 +2287,19 @@ Digit{1,2} ":" Digit{2} ( ":" Digit{2} ( ":" Digit{3} )? )?
  <itemizedlist>
  <listitem>
  <para><literal>l&lt;x&lt;u</literal> means 
- <literal>let t=x in l&lt;t &amp;&amp; t&lt;u</literal>,</para>
+ <literal>let (t=x) l&lt;t &amp;&amp; t&lt;u</literal>,</para>
  </listitem>
  <listitem>
  <para><literal>l&lt;=x&lt;u</literal> means 
- <literal>let t=x in l&lt;=t &amp;&amp; t&lt;u</literal>,</para>
+ <literal>let (t=x) l&lt;=t &amp;&amp; t&lt;u</literal>,</para>
  </listitem>
  <listitem>
  <para><literal>l&lt;x&lt;=u</literal> means 
- <literal>let t=x in l&lt;t &amp;&amp; t&lt;=u</literal>, and</para>
+ <literal>let (t=x) l&lt;t &amp;&amp; t&lt;=u</literal>, and</para>
  </listitem>
  <listitem>
  <para><literal>l&lt;=x&lt;=u</literal> means 
- <literal>let t=x in l&lt;=t &amp;&amp; t&lt;=u</literal></para>
+ <literal>let (t=x) l&lt;=t &amp;&amp; t&lt;=u</literal></para>
  </listitem>
  </itemizedlist>