Update distributed doc (JuliaLang/julia#45368)

* point out Julia versions should be the same

Using addprocs to add workers on remote machines can fail when Julia versions differ because serialization is not guaranteed to be backward compatible. 
This points this out explicitly in the manual.

* updated docstrings for `exename` flag in addprocs

Another location where a warning about the necessity of matching Julia versions across worker processes might be helpful.

* fixed reference in manual

Co-authored-by: Valentin Churavy <[email protected]>

Author: Naikless

src/managers.jl

@@ -107,7 +107,9 @@ addprocs([
   processes. Default is `false`.
 
 * `exename`: name of the `julia` executable. Defaults to `"\$(Sys.BINDIR)/julia"` or
-  `"\$(Sys.BINDIR)/julia-debug"` as the case may be.
+  `"\$(Sys.BINDIR)/julia-debug"` as the case may be. It is recommended that a common Julia 
+  version is used on all remote machines because serialization and code distribution might 
+  fail otherwise.
 
 * `exeflags`: additional flags passed to the worker processes.