changelog: Update through HEAD.

The great-grandparent commit, "a3ae74 'main': Fix the last commit's bug
concerning parameter elision not happening in redirects in command
position.", is not added because it's not a change with respect
to 0.7.1.

Author: danielshahaf

changelog.md

@@ -1,5 +1,8 @@
 # Changes in HEAD
 
+- Document `$ZSH_HIGHLIGHT_MAXLENGTH`.
+  [#698]
+
 - Redirection operators (e.g., `<` and `>`) are now highlighted by default
   [#646]