io: clarify Pipe docs
io: clarify Pipe docs

Fixes #14139.

Change-Id: I6d2181720c38582b3d2160e94c7593a6cb4fc60f
Reviewed-on: https://go-review.googlesource.com/31321
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

codestation/go,polyverse-security/go,CAFxX/go,goodlang/good,cloudflare/go,ImJasonH/go,linux-on-ibm-z/go,CAFxX/go,akutz/go,goodlang/good,josharian/go,ImJasonH/go,Cofyc/go,codestation/go,goodlang/good,josharian/go,derekmarcotte/go,linux-on-ibm-z/go,derekmarcotte/go,CAFxX/go,vsekhar/elastic-go,akutz/go,nawawi/go,nawawi/go,josharian/go,Cofyc/go,codestation/go,Cofyc/go,polyverse-security/go,linux-on-ibm-z/go,CAFxX/go,ImJasonH/go,vsekhar/elastic-go,golang/go,momchil-velikov/go,josharian/go,polyverse-security/go,ImJasonH/go,cloudflare/go,goodlang/good,akutz/go,nawawi/go,suncycheng/go,goodlang/good,momchil-velikov/go,linux-on-ibm-z/go,derekmarcotte/go,CAFxX/go,Cofyc/go,derekmarcotte/go,polyverse-security/go,ImJasonH/go,polyverse-security/go,Cofyc/go,goodlang/good,nawawi/go,momchil-velikov/go,nawawi/go,derekmarcotte/go,suncycheng/go,codestation/go,josharian/go,derekmarcotte/go,momchil-velikov/go,polyverse-security/go,momchil-velikov/go,cloudflare/go,nawawi/go,vsekhar/elastic-go,nawawi/go,cloudflare/go,vsekhar/elastic-go,ImJasonH/go,codestation/go,josharian/go,polyverse-security/go,Cofyc/go,golang/go,golang/go,cloudflare/go,goodlang/good,akutz/go,CAFxX/go,ImJasonH/go,golang/go,momchil-velikov/go,cloudflare/go,suncycheng/go,vsekhar/elastic-go,suncycheng/go,goodlang/good,polyverse-security/go,nawawi/go,linux-on-ibm-z/go,josharian/go,linux-on-ibm-z/go,Cofyc/go,josharian/go,cloudflare/go,codestation/go,momchil-velikov/go,ImJasonH/go,momchil-velikov/go,golang/go,derekmarcotte/go,CAFxX/go,CAFxX/go,akutz/go,vsekhar/elastic-go,suncycheng/go,suncycheng/go,codestation/go,vsekhar/elastic-go,akutz/go,suncycheng/go,Cofyc/go,akutz/go,linux-on-ibm-z/go,codestation/go,golang/go,cloudflare/go,derekmarcotte/go,christopher-henderson/Go,vsekhar/elastic-go,akutz/go,linux-on-ibm-z/go,golang/go,golang/go,suncycheng/go