Merge pull request #2991 from vespa-engine/arnej/document-unpack-bits

add documentation for unpack_bits macro

en/reference/ranking-expressions.html

@@ -962,6 +962,42 @@ <h3 id="non-primitive-functions">Non-primitive functions</h3>
       <p>The hyperbolic tangent of each element.</p>
     </td>
   </tr>
+  <tr>
+    <th>unpack_bits(t)</th>
+    <td>
+      <p id="unpack-bits">unpacks bits from int8 input to 8 times as many floats</p>
+      <p>The innermost indexed dimension will expand to have 8 times as many cells,
+         each with a float value of either 0.0 or 1.0 determined by one bit in
+         the 8-bit input value.  Comparable to <code>numpy.unpackbits</code>
+         which gives the same basic functionality.
+         A minimal input such as <code>tensor&lt;int8&gt;(x[1]):[9]</code>
+         would give output <code>tensor&lt;float&gt;(x[8]):[0,0,0,0,1,0,0,1]</code>
+         (default bit-order is big-endian).
+         As a very complex example, an input with type <code>tensor&lt;int8&gt;(foo{},x[3],y[11],z{})</code>
+         will produce output with type <code>tensor&lt;float&gt;(foo{},x[3],y[88],z{})</code>
+         where "foo", "x" and "z" are unchanged, as "y" is the innermost indexed dimension.
+      </p>
+    </td>
+  </tr>
+  <tr>
+    <th>unpack_bits(t, cell_type)</th>
+    <td>
+      <p id="unpack-bits-cell-type">unpacks bits from int8 input to 8 times as many values</p>
+      <p>Same as above, but with optionally different cell_type (could be <code>double</code>
+         for example, if you will combine the output with other tensors using
+         double).
+      </p>
+    </td>
+  </tr>
+  <tr>
+    <th>unpack_bits(t, cell_type, endian)</th>
+    <td>
+      <p id="unpack-bits-cell-type-endian">unpacks bits from int8 input to 8 times as many values</p>
+      <p>Same as above, but also optionally different endian for the bits; must be
+         either <code>big</code> (default) or <code>little</code>.
+      </p>
+    </td>
+  </tr>
   <tr>
     <th>xw_plus_b(x, w, b, dim)</th>
     <td>