Update index.md

Author: yebai

docs/src/index.md

@@ -10,15 +10,21 @@ public interface of Libtask.jl.
 They divide neatly into two kinds of functions: those which are used to manipulate
 [`TapedTask`](@ref)s, and those which are intended to be used _inside_ a
 [`TapedTask`](@ref).
-First, manipulation of [`TapedTask`](@ref)s:
+
+## Manipulation of [`TapedTask`](@ref)s:
 ```@docs; canonical=true
 Libtask.consume
 Base.copy(::Libtask.TapedTask)
 Libtask.set_taped_globals!
+```
+
+An opt-in mechanism marks functions that might contain `Libtask.produce` statements. 
+
+```@docs; canonical=true
 Libtask.might_produce(::Type{<:Tuple})
 ```
 
-Functions for use inside a [`TapedTask`](@ref)s are:
+## Functions for use inside a [`TapedTask`](@ref)s:
 ```@docs; canonical=true
 Libtask.produce
 Libtask.get_taped_globals