CDRIVER-901 document bulk write error domains

Author: ajdavis

doc/bulk.page

@@ -74,6 +74,7 @@
   "writeConcernErrors" : [] }
 
 Error: E11000 duplicate key error index: test.test.$_id_ dup key: { : 1 }]]></code></synopsis>
+    <p>The <code xref="bson:bson_error_t">bson_error_t</code> domain is <code>MONGOC_ERROR_COMMAND</code> and its code is 11000. </p>
   </section>
 
   <section id="bulk-write-concern">
@@ -94,6 +95,7 @@ Error: E11000 duplicate key error index: test.test.$_id_ dup key: { : 1 }]]></co
 ] }
 
 Error: waiting for replication timed out]]></code></synopsis>
+    <p>The <code xref="bson:bson_error_t">bson_error_t</code> domain is <code>MONGOC_ERROR_WRITE_CONCERN</code> if there are write concern errors and no write errors. Write errors indicate failed operations, so they take precedence over write concern errors, which mean merely that the write concern is not satisfied <em>yet</em>.</p>
   </section>
 
   <section id="bulk-spec">