[libc][docs] Update the fuzzing doc to better reflect the current state.

[sivachandra]

A minor tweak.

libc/docs/fuzzing.rst

-Fuzzing for LLVM-libc
----------------------
+Fuzzing for LLVM-libc functions
+===============================
 
-Fuzzing tests are used to ensure quality and security of LLVM-libc
-implementations.
+Fuzz tests are used to ensure quality and security of LLVM-libc implementations.
+All fuzz tests live under the directory named ``fuzzing``. Within this
+directory, the fuzz test for a libc function lives in the same nested directory
+as its implementation in the toplevel ``src`` directory. The build target
+``libc-fuzzer`` builds all fuzz tests (but does not run them).
 
-Each fuzzing test lives under the fuzzing directory in a subdirectory
-corresponding with the src layout.
+Types of fuzz tests
+===================
 
-Currently we use system libc for functions that have yet to be implemented,
-however as they are implemented the fuzzers will be changed to use our
-implementation to increase coverage for testing.
-
-Fuzzers will be run on `oss-fuzz <https://github.com/google/oss-fuzz>`_ and the
-check-libc target will ensure that they build correctly.
+As of this writing, there are two different kinds of fuzz tests. One kind are
+the traditional fuzz tests which test one function at a time and only that
+particular function. The other kind of tests are what we call as the
+differential fuzz tests. These tests compare the behavior of LLVM libc
+implementations with the behavior of the corresponding functions from the system
+libc.