Add more documentation on the LLVM codegen

Add the required LLVM version (3.3, according to comments in the source)
and note the performance relative to the other codegens (summarized from
the benchmarks in Brian Leibig's project report).

doc/guide/src/LLVMCodegen.adoc

@@ -6,6 +6,15 @@ The <:LLVMCodegen:> is a <:Codegen:code generator> that translates the
 further optimized and compiled to native object code by the <:LLVM:>
 toolchain.
 
+It requires <:LLVM:> version 3.3 or greater to be installed.
+
+In benchmarks performed on the <:RunningOnAMD64:AMD64> architecture,
+code size with this generator is usually slightly smaller than either
+the <:AMD64Codegen:native> or the <:CCodegen:C> code generators. Compile
+time is worse than <:AMD64Codegen:native>, but slightly better than
+<:CCodegen:C>. Run time is often better than either <:AMD64Codegen:native>
+or <:CCodegen:C>.
+
 == Implementation ==
 
 * <!ViewGitFile(mlton,master,mlton/codegen/llvm-codegen/llvm-codegen.sig)>