Insert the GC preserve into the expanded docs (JuliaLang#51884)

The expanded form is not fully correct since it misses the preservation
of
the return value from `cconvert`. The text below talks about the fact
that
`cconvert` should do the allocations necessary, but the expanded code is
incorrect for any `cconvert` that actually does this.

doc/src/manual/calling-c-and-fortran-code.md

@@ -253,10 +253,14 @@ to the specified type. For example, the following call:
 will behave as if it were written like this:
 
 ```julia
-@ccall "libfoo".foo(
- Base.unsafe_convert(Int32, Base.cconvert(Int32, x))::Int32,
- Base.unsafe_convert(Float64, Base.cconvert(Float64, y))::Float64
+c_x = Base.cconvert(Int32, x)
+c_y = Base.cconvert(Float64, y)
+GC.@preserve c_x c_y begin
+ @ccall "libfoo".foo(
+ Base.unsafe_convert(Int32, c_x)::Int32,
+ Base.unsafe_convert(Float64, c_y)::Float64
  )::Cvoid
+end
 ```
 
 [`Base.cconvert`](@ref) normally just calls [`convert`](@ref), but can be defined to return an