Factor logic into registry, make matches cell-based and document

typings/xterm.d.ts

@@ -473,8 +473,26 @@ declare module 'xterm' {
     /**
      * (EXPERIMENTAL) Registers a character joiner, allowing custom sequences of
      * characters to be rendered as a single unit. This is useful in particular
-     * for rendering ligatures. NOTE: character joiners are only used by the
-     * canvas renderer.
+     * for rendering ligatures and graphemes, among other things.
+     * 
+     * Each registered character joiner is called with a string of text
+     * representing a portion of a line in the terminal that can be rendered as
+     * a single unit. The joiner must return a sorted array, where each entry is
+     * itself an array of length two, containing the start (inclusive) and end
+     * (exclusive) index of a substring of the input that should be rendered as
+     * a single unit. When multiple joiners are provided, the results of each
+     * are collected. If there are any overlapping substrings between them, they
+     * are combined into one larger unit that is drawn together.
+     * 
+     * All character joiners that are registered get called every time a line is
+     * rendered in the terminal, so it is essential for the handler function to
+     * run as quickly as possible to avoid slowdowns when rendering. Similarly,
+     * joiners should strive to return the smallest possible substrings to
+     * render together, since they aren't drawn as optimally as individual
+     * characters.
+     * 
+     * NOTE: character joiners are only used by the canvas renderer.
+     * 
      * @param handler The function that determines character joins. It is called
      * with a string of text that is eligible for joining and returns an array
      * where each entry is an array containing the start (inclusive) and end