docs: document using methods in the parser (fixes #3462)

Author: josdejong

docs/expressions/syntax.md

@@ -1,6 +1,6 @@
 # Expression syntax
 
-This page describes the syntax of expression parser of math.js. It describes
+This page describes the syntax of the expression parser of math.js. It describes
 how to work with the available data types, functions, operators, variables,
 and more.
 
@@ -249,6 +249,26 @@ Note that math.js embodies a preference for the operator forms, in that calling
 `simplify` (see [Algebra](./algebra.md)) converts any instances of the function
 form into the corresponding operator.
 
+## Methods
+
+Some data types have methods which can be used in the parser using the dot notation, for example:
+
+```js
+const parser = math.parser()
+
+parser.evaluate('a = 1 m') // Unit 1 m
+parser.evaluate('a.toNumber("mm")') // 1000
+```
+
+Or a method on a Matrix or Array:
+
+```js
+const parser = math.parser()
+
+parser.evaluate('M = [4, 9, 25]') // Matrix [4, 9, 25]
+parser.evaluate('M.map(sqrt)')    // Matrix [2, 3, 5]
+```
+
 ## Map and forEach
 
 The `map` and `forEach` functions can be used to apply a callback function to each element of an array or matrix.