[libfuzzer] Add a note to the documentation about AFL and max_len parameter.

testing/libfuzzer/efficient_fuzzer.md

 # Efficient Fuzzer
 
 This document describes ways to determine your fuzzer efficiency and ways 
 to improve it.
 
 ## Overview
 
 Being a coverage-driven fuzzer, libFuzzer considers a certain input *interesting*
 if it results in new coverage. The set of all interesting inputs is called 
 *corpus*. 
@@ skipping 56 matching lines @@
 2) Increase the value defined on previous step. Check its influence on execution
 speed of fuzzer. If speed doesn't drop significantly for long inputs, it is fine
 to have some bigger value for `-max_len`.
 
 In general, bigger `-max_len` value gives better coverage. Coverage is main
 priority for fuzzing. However, low execution speed may result in waste of
 resources used for fuzzing. If large inputs make fuzzer too slow you have to
 adjust value of `-max_len` and find a trade-off between coverage and execution
 speed.
 
+*Note:* ClusterFuzz runs two different fuzzing engines (**LibFuzzer** and
+**AFL**) using the same target functions. AFL doesn't support `-max_len`
+parameter and may provide input of any length to the target. If your target has
+an input length limit that you would like to *strictly enforce*, it's
+recommended to add a sanity check to the beginning of your target function:
+
+```
+if (size > kSizeLimit)
+  return 0;
+```
+
+For more information check out the discussion in [issue 638836].
+
 
 ## Corpus Size
 
 After running for a while the fuzzer would reach a plateau and won't discover
 new interesting input. Corpus for a reasonably complex functionality
 should contain hundreds (if not thousands) of items.
 
 Too small corpus size indicates some code barrier that
 libFuzzer is having problems penetrating. Common cases include: checksums,
 magic numbers etc. The easiest way to diagnose this problem is to generate a 
@@ skipping 128 matching lines @@
 }
 ```
 
 Please note that `dict` parameter should be provided [separately](#Fuzzer-Dictionary).
 Other options may be passed through `libfuzzer_options` property.
 
 
 [AFL]: http://lcamtuf.coredump.cx/afl/
 [ClusterFuzz status]: clusterfuzz.md#Status-Links
 [Corpus GCS Bucket]: https://goto.google.com/libfuzzer-clusterfuzz-corpus
+[issue 638836]: https://bugs.chromium.org/p/chromium/issues/detail?id=638836