Rephrase displayMode description to be clearer (#648)

* Clarify meaning of true and false
* Clarify inline vs. block

src/Settings.js

@@ -9,8 +9,11 @@ const utils = require("./utils");
  * The main Settings object
  *
  * The current options stored are:
- *  - displayMode: Whether the expression should be typeset by default in
- *                 textstyle or displaystyle (default false)
+ *  - displayMode: Whether the expression should be typeset as inline math
+ *                 (false, the default), meaning that the math starts in
+ *                 \textstyle and is placed in an inline-block); or as display
+ *                 math (true), meaning that the math starts in \displaystyle
+ *                 and is placed in a block with vertical margin.
  */
 function Settings(options) {
     // allow null options