net/http: document Dir behavior with symlinks
net/http: document Dir behavior with symlinks

Based on CL 229377.

Change-Id: I016eec20672c909e8fabe799c277f4d2540fc555
Reviewed-on: https://go-review.googlesource.com/c/go/+/238698
Run-TryBot: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>

nawawi/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,golang/go,golang/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go