change header_length type, change block_producer_time_nanos description

Author: jherrera-jump

proposals/0307-add-block-header.md

@@ -75,7 +75,7 @@ entry batch in the block.
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          version          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-|       header_length       |
+| header_length | <- 16 bits
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | block_producer_time_nanos |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
@@ -97,12 +97,11 @@ to.
 - `version: u64` is a positive integer which changes anytime a change is made to
 the header. The initial version will be 1.
 
-- `header_length: u64` is the length of the rest of the header in bytes (i.e.
+- `header_length: u16` is the length of the rest of the header in bytes (i.e.
 not including the `block_header_flag`, `version`, and `header_length` fields).
 
 - `block_producer_time_nanos: u64` is a nanosecond UNIX timestamp representing
-the time when the block producer became leader and started constructing the
-block.
+the time when the block producer started constructing the block.
 
 - `block_user_agent: [u8; 256]` is a string that provides identifying
 information about the block producer.