List bn_div fuzzer in documentation.

c951e5560b added it back in the build, but it should be listed in
documentation too. Picked 384 bytes on the assumption that the largest
specialisation that we're likely to have is 1024-bit for RSA 2048.

Change-Id: I13fb46dceedb7a62616db6a2b39634a0b6fed508
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/38444
Commit-Queue: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/FUZZING.md b/FUZZING.md
index 200f7a5..c2f802f 100644
--- a/FUZZING.md
+++ b/FUZZING.md
@@ -26,6 +26,7 @@
 
 | Test          | `max_len` value |
 |---------------|-----------------|
+| `bn_div`      | 384             |
 | `bn_mod_exp`  | 4096            |
 | `cert`        | 10000           |
 | `client`      | 20000           |