[LibFuzzer] Improve comments on `CounterToFeature()` function.

[delcypher]

Summary

This is based on discussion in https://reviews.llvm.org/D40376 .

The comments try to explain the reason for the current implementation
and note that it might change in the future, so clients should not
rely on this particular implementation.

[kcc]

LGTM, thanks!
Do you need us to land it?
If so, Matt will do it.

[delcypher]

In D40565#937827, @kcc wrote:

LGTM, thanks!
Do you need us to land it?
If so, Matt will do it.

I can commit this. By the way the code in this function isn't properly formatted. I decided not to change it in this patch though because I didn't want to mix the two things together.

compiler-rt/trunk/lib/fuzzer/FuzzerTracePC.h

if (LargeType Bundle = *reinterpret_cast<const LargeType *>(P))
           Handle8bitCounter(FirstFeature, P - Begin + I, V);
 
   // Iterate by 1 byte until the end.
   for (; P < End; P++)
     if (uint8_t V = *P)
       Handle8bitCounter(FirstFeature, P - Begin, V);
 }
 
-// Given a non-zero Counters returns a number in [0,7].
+// Given a non-zero Counter returns a number in the range [0,7].
 template<class T>
 unsigned CounterToFeature(T Counter) {
+    // Returns a feature number by placing Counters into buckets as illustrated
+    // below.
+    //
+    // Counter bucket: [1] [2] [3] [4-7] [8-15] [16-31] [32-127] [128+]
+    // Feature number:  0   1   2    3     4       5       6       7
+    //
+    // This is a heuristic taken from AFL (see
+    // http://lcamtuf.coredump.cx/afl/technical_details.txt).
+    //
+    // This implementation may change in the future so clients should
+    // not rely on it.
     assert(Counter);
     unsigned Bit = 0;
     /**/ if (Counter >= 128) Bit = 7;
     else if (Counter >= 32) Bit = 6;
     else if (Counter >= 16) Bit = 5;
     else if (Counter >= 8) Bit = 4;
     else if (Counter >= 4) Bit = 3;
     else if (Counter >= 3) Bit = 2;