docs: update bodies config (#3553)

book/run/config.md

@@ -90,14 +90,16 @@ downloader_request_limit = 200
 # A lower value means more frequent disk I/O (writes), but also
 # lowers memory usage.
 downloader_stream_batch_size = 10000
-# The maximum amount of blocks to keep in the internal buffer of the downloader.
+# The size of the internal block buffer in bytes.
 #
 # A bigger buffer means that bandwidth can be saturated for longer periods,
 # but also increases memory consumption.
 #
 # If the buffer is full, no more requests will be made to peers until
 # space is made for new blocks in the buffer.
-downloader_max_buffered_blocks = 42949
+#
+# Defaults to around 4GB.
+downloader_max_buffered_blocks_size_bytes = 4294967296
 # The minimum and maximum number of concurrent requests to have in flight at a time.
 #
 # The downloader uses these as best effort targets, which means that the number