Tweaks to build_callable and docs

Author: willtebbutt

docs/src/internals.md

@@ -14,4 +14,5 @@ Libtask.BasicBlockCode
 Libtask.opaque_closure
 Libtask.misty_closure
 Libtask.optimise_ir!
+Libtask.build_callable
 ```

src/copyable_task.jl

@@ -74,8 +74,7 @@ will derive it from scratch (derive the IR + compile it etc).
 function build_callable(sig::Type{<:Tuple})
     key = CacheKey(Base.get_world_counter(), sig)
     if haskey(mc_cache, key)
-        v = fresh_copy(mc_cache[key])
-        return v
+        return fresh_copy(mc_cache[key])
     else
         ir = Base.code_ircode_by_type(sig)[1][1]
         bb, refs, types = derive_copyable_task_ir(BBCode(ir))