Add some missing docstrings in manual (JuliaLang/julia#44642)

Author: Saransh-cpp

docs/src/index.md

@@ -1,4 +1,5 @@
 # [Distributed Computing](@id man-distributed)
+Tools for distributed parallel processing.
 
 ```@docs
 Distributed.addprocs
@@ -12,6 +13,7 @@ Distributed.interrupt
 Distributed.myid
 Distributed.pmap
 Distributed.RemoteException
+Distributed.ProcessExitedException
 Distributed.Future
 Distributed.RemoteChannel
 Distributed.fetch(::Distributed.Future)