Update libFuzzer documentation for -fsanitize=fuzzer-no-link flag

[kcc]

LGTM with a nit

docs/LibFuzzer.rst

108

Then libFuzzer can ...

docs/LibFuzzer.rst

 * It must be fast. Try avoiding cubic or greater complexity, logging, or excessive memory consumption.
 * Ideally, it should not modify any global state (although that's not strict).
 * Usually, the narrower the target the better. E.g. if your target can parse several data formats, split it into several targets, one per format.
 
 
 Fuzzer Usage
 ------------
 
-Very recent versions of Clang (> April 20 2017) include libFuzzer,
+Very recent versions of Clang (after April 20 2017) include libFuzzer,
 and no installation is necessary.
 In order to fuzz your binary, use the `-fsanitize=fuzzer` flag during the compilation::
 
    clang -fsanitize=fuzzer,address mytarget.c
 
+This will perform the necessary instrumentation, as well as linking in libFuzzer
+library.
+Note that linking in libFuzzer defines the ``main`` symbol.
+If modifying ``CFLAGS`` of a large project, which also compiles executables
+requiring their own ``main`` symbol, it may be desirable to request just the
+instrumentation without linking::
+
+   clang -fsanitize=fuzzer-no-link mytarget.c
+
+Then library can still be linked to the desired driver by passing in
+``-fsanitize=fuzzer`` during the linking stage.
+
 Otherwise, build the libFuzzer library as a static archive, without any sanitizer
 options. Note that the libFuzzer library contains the ``main()`` function:
 
 .. code-block:: console
 
   svn co http://llvm.org/svn/llvm-project/llvm/trunk/lib/Fuzzer  # or git clone https://chromium.googlesource.com/chromium/llvm-project/llvm/lib/Fuzzer
   ./Fuzzer/build.sh  # Produces libFuzzer.a