runtime/trace: improve package doc
runtime/trace: improve package doc

Explaining how to enable tracing.

Change-Id: Ic8391cb290742b4dc1efab15deba85853e182b4d
Reviewed-on: https://go-review.googlesource.com/59612
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

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