Clarified use of unions in C++.

Change-Id: I9654e0c6a45457c8e150f07dd5f7b39539266f9e

docs/html/md__cpp_usage.html

@@ -79,7 +79,7 @@ $(document).ready(function(){initNavTree('md__cpp_usage.html','');});
 <p>We have now serialized the non-scalar components of of the monster example, so we could create the monster something like this:</p>
 <div class="fragment"><div class="line"><span class="keyword">auto</span> mloc = CreateMonster(fbb, &amp;vec, 150, 80, name, inventory, Color_Red, 0, Any_NONE);</div>
 </div><!-- fragment --><p>Note that we're passing <code>150</code> for the <code>mana</code> field, which happens to be the default value: this means the field will not actually be written to the buffer, since we'll get that value anyway when we query it. This is a nice space savings, since it is very common for fields to be at their default. It means we also don't need to be scared to add fields only used in a minority of cases, since they won't bloat up the buffer sizes if they're not actually used.</p>
-<p>We do something similarly for the union field <code>test</code> by specifying a <code>0</code> offset and the <code>NONE</code> enum value (part of every union) to indicate we don't actually want to write this field. You can use <code>0</code> also as a default for other non-scalar types, such as strings, vectors and tables.</p>
+<p>We do something similarly for the union field <code>test</code> by specifying a <code>0</code> offset and the <code>NONE</code> enum value (part of every union) to indicate we don't actually want to write this field. You can use <code>0</code> also as a default for other non-scalar types, such as strings, vectors and tables. To pass an actual table, pass a preconstructed table as <code>mytable.Union()</code> that corresponds to union enum you're passing.</p>
 <p>Tables (like <code>Monster</code>) give you full flexibility on what fields you write (unlike <code>Vec3</code>, which always has all fields set because it is a <code>struct</code>). If you want even more control over this (i.e. skip fields even when they are not default), instead of the convenient <code>CreateMonster</code> call we can also build the object field-by-field manually:</p>
 <div class="fragment"><div class="line">MonsterBuilder mb(fbb);</div>
 <div class="line">mb.add_pos(&amp;vec);</div>