doc: misc updates (#30837)

* include docstrings from #30716, #30745 and #30747
in the manual, and misc cross references.

* fix doctest for =

src/clusterserialize.jl

@@ -251,7 +251,7 @@ end
     clear!(syms, pids=workers(); mod=Main)
 
 Clears global bindings in modules by initializing them to `nothing`.
-`syms` should be of type `Symbol` or a collection of `Symbol`s . `pids` and `mod`
+`syms` should be of type [`Symbol`](@ref) or a collection of `Symbol`s . `pids` and `mod`
 identify the processes and the module in which global variables are to be
 reinitialized. Only those names found to be defined under `mod` are cleared.