Some small updates to the Span<T> design document (#1955)
Some small updates to the Span<T> design document (#1955)

I received some comments on parts of the spec being outdated. These are not comprehensive fixes, but rather those that become confusing at this point.
tarekgh/corefxlab,adamsitnik/corefxlab,stephentoub/corefxlab,KrzysztofCwalina/corefxlab,ahsonkhan/corefxlab,stephentoub/corefxlab,adamsitnik/corefxlab,stephentoub/corefxlab,whoisj/corefxlab,ahsonkhan/corefxlab,joshfree/corefxlab,ericstj/corefxlab,whoisj/corefxlab,stephentoub/corefxlab,dotnet/corefxlab,ericstj/corefxlab,ericstj/corefxlab,ericstj/corefxlab,KrzysztofCwalina/corefxlab,dotnet/corefxlab,benaadams/corefxlab,ericstj/corefxlab,stephentoub/corefxlab,benaadams/corefxlab,joshfree/corefxlab,ericstj/corefxlab,stephentoub/corefxlab,tarekgh/corefxlab