net/http: clarify when it is safe to reuse a request
net/http: clarify when it is safe to reuse a request

The godoc for RoundTrip already specifies when it's ok to reuse a
request that contains a body: the caller must wait until RoundTrip
calls Close on Request.Body.

This CL adds a small clarification: If the request does not have a
body, it can be reused as long as the caller does not mutate the
Request until RoundTrip fails or the Response.Body is closed.

Fixes #19653

Change-Id: I56652a9369978d11650e2e6314104831c2ce5e78
Reviewed-on: https://go-review.googlesource.com/75671
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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