Recommend X-Modhash usage in the API docs.

r2/r2/lib/validator/validator.py

@@ -815,7 +815,7 @@ class VModhash(Validator):
 
     def param_docs(self):
         return {
-            self.param: 'a [modhash](#modhashes)',
+            '%s / X-Modhash header' % self.param: 'a [modhash](#modhashes)',
         }
 
 class VVotehash(Validator):

r2/r2/templates/apihelp.html

@@ -140,6 +140,9 @@
     or in the&#32;<code>Listing</code>&#32;structures of several other API
     calls.</p>
 
+    <p>The preferred way to send a modhash is to include an
+    &#32;<code>X-Modhash</code>&#32;custom HTTP header with your requests.</p>
+
     <h3 id="fullnames">fullnames</h3>
 
     <p>A fullname is a combination of a thing's type