net/http: better documentation for Transport
net/http: better documentation for Transport

Mention that:
- connection pooling is enabled by default,
- the Transport is safe for concurrent use, and
- the Client type should be used for high-level stuff.

Change-Id: Idfd8cc852e733c44211e77cf0e22720b1fdca39b
Reviewed-on: https://go-review.googlesource.com/18273
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

momchil-velikov/go,binarycrusader/go,codestation/go,polyverse-security/go,CAFxX/go,Cofyc/go,josharian/go,golang/go,binarycrusader/go,sndnvaps/go,cyberroadie/go,Samurais/go,ganboing/go-esx,polyverse-security/go,suncycheng/go,derekmarcotte/go,ImJasonH/go,Samurais/go,suncycheng/go,Endika/go,akutz/go,Endika/go,polyverse-security/go,Samurais/go,codestation/go,vanloswang/go,mwhudson/go,momchil-velikov/go,nawawi/go,vanloswang/go,4ad/go,4ad/go,vanloswang/go,cloudflare/go,momchil-velikov/go,derekmarcotte/go,josharian/go,akutz/go,vsekhar/elastic-go,sndnvaps/go,derekmarcotte/go,josharian/go,miolini/go,josharian/go,linux-on-ibm-z/go,codestation/go,mwhudson/go,codestation/go,theif519/go,ImJasonH/go,ImJasonH/go,goodlang/good,vanloswang/go,ganboing/go-esx,CAFxX/go,ImJasonH/go,josharian/go,ganboing/go-esx,binarycrusader/go,golang/go,linux-on-ibm-z/go,linux-on-ibm-z/go,nawawi/go,suncycheng/go,codestation/go,akutz/go,theif519/go,Cofyc/go,cyberroadie/go,ganboing/go-esx,CAFxX/go,cyberroadie/go,momchil-velikov/go,miolini/go,theif519/go,4ad/go,CAFxX/go,Cofyc/go,miolini/go,sndnvaps/go,christopher-henderson/Go,binarycrusader/go,cloudflare/go,goodlang/good,Endika/go,Cofyc/go,suncycheng/go,goodlang/good,binarycrusader/go,cloudflare/go,mwhudson/go,golang/go,vanloswang/go,Samurais/go,CAFxX/go,ImJasonH/go,miolini/go,cloudflare/go,4ad/go,ganboing/go-esx,golang/go,ganboing/go-esx,suncycheng/go,linux-on-ibm-z/go,goodlang/good,linux-on-ibm-z/go,4ad/go,akutz/go,suncycheng/go,sndnvaps/go,polyverse-security/go,Endika/go,4ad/go,miolini/go,linux-on-ibm-z/go,sndnvaps/go,akutz/go,vanloswang/go,vanloswang/go,akutz/go,Samurais/go,miolini/go,suncycheng/go,mwhudson/go,golang/go,cyberroadie/go,nawawi/go,Cofyc/go,suncycheng/go,nawawi/go,cyberroadie/go,Endika/go,cyberroadie/go,ImJasonH/go,CAFxX/go,ImJasonH/go,vsekhar/elastic-go,golang/go,derekmarcotte/go,goodlang/good,derekmarcotte/go,josharian/go,vsekhar/elastic-go,cloudflare/go,CAFxX/go,cloudflare/go,binarycrusader/go,cloudflare/go,ganboing/go-esx,polyverse-security/go,binarycrusader/go,vsekhar/elastic-go,Cofyc/go,sndnvaps/go,golang/go,vsekhar/elastic-go,goodlang/good,vsekhar/elastic-go,akutz/go,momchil-velikov/go,momchil-velikov/go,miolini/go,sndnvaps/go,nawawi/go,Endika/go,Samurais/go,mwhudson/go,mwhudson/go,vsekhar/elastic-go,josharian/go,cyberroadie/go,Cofyc/go,Endika/go,theif519/go,goodlang/good,josharian/go,codestation/go,Endika/go,linux-on-ibm-z/go,nawawi/go,ganboing/go-esx,ImJasonH/go,cyberroadie/go,Samurais/go,derekmarcotte/go,Cofyc/go,polyverse-security/go,theif519/go,theif519/go,mwhudson/go,momchil-velikov/go,theif519/go,polyverse-security/go,sndnvaps/go,vsekhar/elastic-go,akutz/go,cloudflare/go,goodlang/good,mwhudson/go,nawawi/go,vanloswang/go,codestation/go,4ad/go,derekmarcotte/go,golang/go,Samurais/go,binarycrusader/go,derekmarcotte/go,linux-on-ibm-z/go,theif519/go,nawawi/go,4ad/go,momchil-velikov/go,polyverse-security/go,CAFxX/go,miolini/go,codestation/go