changelog: Document #670 as a known issue.
changelog: Document #670 as a known issue.

phy1729/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,x-way/zsh-syntax-highlighting,zsh-users/zsh-syntax-highlighting,zsh-users/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,x-way/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting