Document ZSH_HIGHLIGHT_MAXLENGTH.
Document ZSH_HIGHLIGHT_MAXLENGTH.

Fixes #698.

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