runtime: avoid redundant zeroing of hiter
runtime: avoid redundant zeroing of hiter

The compiler and reflect already zero hiter before mapiterinit.

While here expand the documentation for mapiterinit.

Change-Id: I78b05d4d14bf78e8091e5353cdac80ffed30ca1e
Reviewed-on: https://go-review.googlesource.com/60673
Run-TryBot: Martin Möhrmann <9c82b83f348009b39856043bb9ed3ef41e171aeb@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

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