FFI: Add 64 bit bitwise operations.

doc/ext_ffi_semantics.html

@@ -730,6 +730,22 @@ You'll have to explicitly convert a 64 bit integer to a Lua
 number (e.g. for regular floating-point calculations) with
 <tt>tonumber()</tt>. But note this may incur a precision loss.</li>
 
+<li><b>64&nbsp;bit bitwise operations</b>: the rules for 64&nbsp;bit
+arithmetic operators apply analogously.<br>
+
+Unlike the other <tt>bit.*</tt> operations, <tt>bit.tobit()</tt>
+converts a cdata number via <tt>int64_t</tt> to <tt>int32_t</tt> and
+returns a Lua number.<br>
+
+For <tt>bit.band()</tt>, <tt>bit.bor()</tt> and <tt>bit.bxor()</tt>, the
+conversion to <tt>int64_t</tt> or <tt>uint64_t</tt> applies to
+<em>all</em> arguments, if <em>any</em> argument is a cdata number.<br>
+
+For all other operations, only the first argument is used to determine
+the output type. This implies that a cdata number as a shift count for
+shifts and rotates is accepted, but that alone does <em>not</em> cause
+a cdata number output.
+
 </ul>
 
 <h3 id="cdata_comp">Comparisons of cdata objects</h3>
@@ -1222,7 +1238,6 @@ value.</li>
 Other missing features:
 </p>
 <ul>
-<li>Bit operations for 64&nbsp;bit types.</li>
 <li>Arithmetic for <tt>complex</tt> numbers.</li>
 <li>Passing structs by value to vararg C&nbsp;functions.</li>
 <li><a href="extensions.html#exceptions">C++ exception interoperability</a>