os/exec: Stdout/Stderr doc cleanup.

Following comments on CL 76320.

Breaks Cmd.Std{out,err} doc into three paragraphs and updates Cmd.Stdin
formatting to match.

Fixes an erroneous reference to Stdin in the output goroutine comment, while
keeping the wording consistent between Stdin and Stdout/Stderr.

Change-Id: I186a0e2d4b85dfb939443a17e62a1eb2ef64b1bf
Reviewed-on: https://go-review.googlesource.com/79595


Reviewed-by: Rob Pike <r@golang.org>