document mutable struct const fields (#55203)

Introduced in #43305

Author: IanButterworth

base/docs/basedocs.jl

@@ -1451,8 +1451,20 @@ kw"struct"
     mutable struct
 
 `mutable struct` is similar to [`struct`](@ref), but additionally allows the
-fields of the type to be set after construction. See the manual section on
-[Composite Types](@ref) for more information.
+fields of the type to be set after construction.
+
+Individual fields of a mutable struct can be marked as `const` to make them immutable:
+
+```julia
+mutable struct Baz
+    a::Int
+    const b::Float64
+end
+```
+!!! compat "Julia 1.8"
+    The `const` keyword for fields of mutable structs requires at least Julia 1.8.
+
+See the manual section on [Composite Types](@ref) for more information.
 """
 kw"mutable struct"