Extra documentation.

Some other reserved bits are repurposed. Also explicitly mention that
bit 20 is zero (formerly RC4_CHAR), so it's not accidentally repurposed
later.

Change-Id: Idc4b32efe089ae7b7295472c4488f75258b7f962
Reviewed-on: https://boringssl-review.googlesource.com/5432
Reviewed-by: Adam Langley <agl@google.com>
1 file changed