Document file descriptors in run (#45710)

base/process.jl

@@ -464,6 +464,9 @@ Run a command object, constructed with backticks (see the [Running External Prog
 section in the manual). Throws an error if anything goes wrong, including the process
 exiting with a non-zero status (when `wait` is true).
 
+The `args...` allow you to pass through file descriptors to the command, and are ordered
+like regular unix file descriptors (eg `stdin, stdout, stderr, FD(3), FD(4)...`).
+
 If `wait` is false, the process runs asynchronously. You can later wait for it and check
 its exit status by calling `success` on the returned process object.