document use of @eval with @Everywhere for broadcast of local variables. (#15960)

Proposed by Ben Arthur (@bjarthur)

Author: amitmurthy

base/docs/helpdb/Base.jl

@@ -144,7 +144,19 @@ download
     @everywhere
 
 Execute an expression on all processes. Errors on any of the processes are collected into a
-`CompositeException` and thrown.
+`CompositeException` and thrown. For example :
+
+    @everywhere bar=1
+
+will define `bar` under module `Main` on all processes.
+
+Unlike `@spawn` and `@spawnat`, `@everywhere` does not capture any local variables. Prefixing
+`@everywhere` with `@eval` allows us to broadcast local variables using interpolation :
+
+    foo = 1
+    @eval @everywhere bar=\$foo
+
+
 """
 :@everywhere
 

doc/stdlib/parallel.rst

@@ -502,7 +502,20 @@ General Parallel Computing Support
 
    .. Docstring generated from Julia source
 
-   Execute an expression on all processes. Errors on any of the processes are collected into a ``CompositeException`` and thrown.
+   Execute an expression on all processes. Errors on any of the processes are collected into a ``CompositeException`` and thrown. For example :
+
+   .. code-block:: julia
+
+       @everywhere bar=1
+
+   will define ``bar`` under module ``Main`` on all processes.
+
+   Unlike ``@spawn`` and ``@spawnat``\ , ``@everywhere`` does not capture any local variables. Prefixing ``@everywhere`` with ``@eval`` allows us to broadcast local variables using interpolation :
+
+   .. code-block:: julia
+
+       foo = 1
+       @eval @everywhere bar=$foo
 
 .. function:: Base.remoteref_id(r::AbstractRemoteRef) -> (whence, id)