Improve macro docstring

Author: penelopeysm

docs/src/index.md

@@ -11,14 +11,14 @@ They divide neatly into two kinds of functions: those which are used to manipula
 [`TapedTask`](@ref)s, and those which are intended to be used _inside_ a
 [`TapedTask`](@ref).
 
-## Manipulation of [`TapedTask`](@ref)s:
+## Manipulation of [`TapedTask`](@ref)s
 ```@docs; canonical=true
 Libtask.consume
 Base.copy(::Libtask.TapedTask)
 Libtask.set_taped_globals!
 ```
 
-## Functions for use inside a [`TapedTask`](@ref)s:
+## Functions for use inside a [`TapedTask`](@ref)s
 ```@docs; canonical=true
 Libtask.produce
 Libtask.get_taped_globals

src/copyable_task.jl

@@ -364,8 +364,14 @@ might_produce(::Type{<:Tuple}) = false
 
 If `f` is a function that may call `Libtask.produce` inside it, then `@might_produce(f)`
 will generate the appropriate methods needed to ensure that `Libtask.might_produce` returns
-`true` for all relevant signatures of `f`. This works even if `f` has methods with keyword
-arguments.
+`true` for **all** relevant signatures of `f`. This works even if `f` has methods with
+keyword arguments.
+
+!!! note
+    Because `@might_produce f` is applied to all possible signatures, there may be
+    performance penalties associated with marking `f` as produceable if it is only
+    applicable to a specific method signature. If performance is critical, please use the
+    [`might_produce`](@ref) function directly.
 
 ```jldoctest might_produce_macro
 julia> # For this demonstration we need to mark `g` as not being inlineable.