Add docstring for fallback fetch(::Any) method (JuliaLang#45008)

We cannot just include `Base.fetch` in the `@docs` block as
some methods should keep being documented in the Distributed stdlib
rather than in Base.

Author: nalimilan

base/task.jl

@@ -351,6 +351,11 @@ function wait(t::Task)
     nothing
 end
 
+"""
+    fetch(x::Any)
+
+Return `x`.
+"""
 fetch(@nospecialize x) = x
 
 """

doc/src/base/parallel.md

@@ -33,6 +33,7 @@ Base.errormonitor
 Base.@sync
 Base.wait
 Base.fetch(t::Task)
+Base.fetch(x::Any)
 Base.timedwait
 
 Base.Condition