Fix documentation with 'soft-float' codegen option

This option doesn't cause software FP routines
to be called it only changes the float ABI.

Additionally, this option is ignored by all targets,
except the ARM eabihf ones.

src/librustc/session/config.rs

@@ -770,7 +770,7 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
     no_vectorize_slp: bool = (false, parse_bool, [TRACKED],
         "don't run LLVM's SLP vectorization pass"),
     soft_float: bool = (false, parse_bool, [TRACKED],
-        "generate software floating point library calls"),
+        "use soft float ABI (*eabihf targets only)"),
     prefer_dynamic: bool = (false, parse_bool, [TRACKED],
         "prefer dynamic linking to static linking"),
     no_integrated_as: bool = (false, parse_bool, [TRACKED],