os/exec: Stdout/Stderr doc cleanup.
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 <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

cloudflare/go,akutz/go,golang/go,CAFxX/go,CAFxX/go,codestation/go,codestation/go,cloudflare/go,codestation/go,Cofyc/go,cloudflare/go,akutz/go,josharian/go,CAFxX/go,CAFxX/go,Cofyc/go,cloudflare/go,codestation/go,cloudflare/go,nawawi/go,nawawi/go,Cofyc/go,cloudflare/go,Cofyc/go,golang/go,golang/go,Cofyc/go,josharian/go,cloudflare/go,nawawi/go,Cofyc/go,nawawi/go,akutz/go,codestation/go,josharian/go,akutz/go,codestation/go,josharian/go,cloudflare/go,josharian/go,CAFxX/go,akutz/go,Cofyc/go,akutz/go,akutz/go,golang/go,josharian/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,akutz/go,Cofyc/go,golang/go,golang/go,josharian/go,codestation/go,nawawi/go,codestation/go,CAFxX/go,christopher-henderson/Go,nawawi/go,CAFxX/go,golang/go,josharian/go