Improve documentation for process interaction using pipes

[mbaz]

Current documentation covers some aspects of launching processes and interacting with them using STDIN and STDOUT. However, Julia has much more powerful capabilities that remain largely undocumented. In particular, concurrent use of the three standard process streams STDIN, STDOUT and STDERR is not documented.

See previous discussion here (and links therein).

In addition, there are several ways to acomplish this kind of iteration. It has been suggested that some of these may be deprecated in favor of pipeline.

These are the suggested tasks:

• Document Pipe() (Deprecate readandwrite and add docs for Pipe #24718)

• Deprecate or document four-argument spawn. Example:

  julia> stdin, stdout, stderr = (Pipe() for _ in 1:3);
  julia> spawn(`cat`, stdin, stdout, stderr)
  

• Deprecate or document use of pipeline with three pipes. Example:

  julia> stdin, stdout, stderr = (Pipe() for _ in 1:3);
  julia> pipeline(`cat`, stdin=stdin, stdout=stdout, stderr=stderr)
  

• Deprecate or document use of pipeline with one pipe. This involves also documenting that the objects returned by open allow access to the pipes (is this also true of spawn?) Example (not supported in v0.6):

  julia> p = open(pipeline(`cat`; stderr=Pipe()), "r+")
  # Use p.{in,out,err} to access
  

[vtjnash]

I think we should consider deprecating some of the calls that take a specific number of pipes, since pipeline is much more general.

[mbaz]

@vtjnash I updated the proposed task list.

[fingolfin]

Note that while Pipe now has a doc string, it does not seem to be part of the manual yet -- or at least I can't find it, see https://docs.julialang.org/en/v1/search/?q=Pipe