Clarify truncation behaviour of get_chunk in docs

docs/PROTOCOL.rst

@@ -388,7 +388,10 @@ bandwidth-intensive request.
 **Response**
 
     The binary block headers, as hexadecimal strings, in order
-    concatenated together.
+    concatenated together.  As many as headers as are available at
+    starting height 2016 * index will be returned; this may range from
+    0 to 2016.
+
 
 
 blockchain.estimatefee

server/session.py

@@ -254,6 +254,26 @@ class ElectrumX(SessionBase):
         '''Returns a dictionary of server features.'''
         return self.env.server_features()
 
+    def block_headers(self, start_height, count):
+        '''Return concatenated block headers as hex for the main chain;
+        count headers starting at start_height.
+
+        start_height and count must be non-negative integers.'''
+        start_height = self.controller.non_negative_integer(start_height)
+        count = self.controller.non_negative_integer(count)
+        return self.controller.block_headers(start_height, count).hex()
+
+    def block_get_chunk(self, index):
+        '''Return a chunk of block headers as a hexadecimal string.
+
+        index: the chunk index'''
+        index = self.controller.non_negative_integer(index)
+        chunk_size = self.coin.CHUNK_SIZE
+        start_height = index * chunk_size
+        count = chunk_size
+
+        return self.controller.block_headers(start_height, count).hex()
+
     def block_get_chunk(self, index):
         '''Return a chunk of block headers as a hexadecimal string.