Make it clear that kwargs is immutable (#35998)

See #35985

Author: mbaz

doc/src/manual/functions.md

@@ -602,9 +602,9 @@ function f(x; y=0, kwargs...)
 end
 ```
 
-Inside `f`, `kwargs` will be a key-value iterator over a named tuple. Named
-tuples (as well as dictionaries with keys of `Symbol`) can be passed as keyword
-arguments using a semicolon in a call, e.g. `f(x, z=1; kwargs...)`.
+Inside `f`, `kwargs` will be an immutable key-value iterator over a named tuple.
+Named tuples (as well as dictionaries with keys of `Symbol`) can be passed as
+keyword arguments using a semicolon in a call, e.g. `f(x, z=1; kwargs...)`.
 
 If a keyword argument is not assigned a default value in the method definition,
 then it is *required*: an [`UndefKeywordError`](@ref) exception will be thrown