Recommend using RawFD instead of the Int returned by fd (#55027)

Helps with #51710

---------

Co-authored-by: Daniel Karrasch <[email protected]>

Author: LilithHafner

base/iostream.jl

@@ -51,6 +51,11 @@ end
 
 Return the file descriptor backing the stream or file. Note that this function only applies
 to synchronous `File`'s and `IOStream`'s not to any of the asynchronous streams.
+
+File descriptors should typically be represented as [`RawFD`](@ref) objects, rather
+than as `Int`s, to ensure that they are properly interpreted by Julia functions.
+
+Note that `RawFD` objects can be passed directly to other languages via the `ccall` interface.
 """
 fd(s::IOStream) = Int(ccall(:jl_ios_fd, Clong, (Ptr{Cvoid},), s.ios))