performance tips docs: add a note when force sepecialization of a function may be required with optional arguments

doc/src/manual/performance-tips.md

@@ -652,6 +652,29 @@
 when argument types are changed, i.e., if `Base.specializations(@which f(...))` contains specializations
 for the argument in question.
 
+Note that in the presence of keywords and optional positional arguments, you may need to force specialization of a method argument
+even though the method is called in the body. This is due to how Julia rewrites method with default arguments. If we consider:
+
+```julia
+g(f, x=1) = f(...)
+```
+
+the method argument `f` is here clearly used within the method `g`. However, julia will rewrite this as:
+
+```julia
+g(f, x) = f(...)
+g(f) = g(f, 1)
+```
+
+With this rewrite, the second method that passes along the default argument value does no longer use `f`, calling `g(f)` may therefore not specialize on `f`.
+Writing the original definition as
+
+```julia
+g(f::F, x=1) where {F} = f(...)
+```
+
+woudld solve this.
+
 ### Write "type-stable" functions
 
 When possible, it helps to ensure that a function always returns a value of the same type. Consider