[docs] Add a brief description of using -fveclib to enable some math library vectorizations (llvm#119215)

Fixes llvm#62283

Author: RKSimon

llvm/docs/Vectorizers.rst

@@ -345,6 +345,16 @@ instruction is available.
       f[i] = floorf(f[i]);
   }
 
+Many of these math functions are only vectorizable if the file has been built
+with a specified target vector library that provides a vector implemention
+of that math function. Using clang, this is handled by the "-fveclib" command
+line option with one of the following vector libraries:
+"accelerate,libmvec,massv,svml,sleef,darwin_libsystem_m,armpl,amdlibm"
+
+.. code-block:: console
+
+   $ clang ... -fno-math-errno -fveclib=libmvec file.c
+
 Partial unrolling during vectorization
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^