Clarify math operations available for interpolation

As reported in hashicorp#2782, the math operations, specifically subtraction,
can cause unexpected behavior when resource or variable names use hyphens.

I added clarification about using spaces with math operators as well as
which operations are available.

Author: clstokes

website/source/docs/configuration/interpolation.html.md

@@ -17,7 +17,7 @@ The interpolation syntax is powerful and allows you to reference
 variables, attributes of resources, call functions, etc.
 
 You can also perform simple math in interpolations, allowing
-you to write expressions such as `${count.index+1}`.
+you to write expressions such as `${count.index + 1}`.
 
 You can escape interpolation with double dollar signs: `$${foo}`
 will be rendered as a literal `${foo}`.
@@ -92,7 +92,7 @@ The supported built-in functions are:
       format. The syntax for the format is standard `sprintf` syntax.
       Good documentation for the syntax can be [found here](http://golang.org/pkg/fmt/).
       Example to zero-prefix a count, used commonly for naming servers:
-      `format("web-%03d", count.index+1)`.
+      `format("web-%03d", count.index + 1)`.
 
   * `formatlist(format, args...)` - Formats each element of a list
       according to the given format, similarly to `format`, and returns a list.
@@ -201,3 +201,34 @@ resource "aws_instance" "web" {
 
 With this, we will build a list of `template_file.web_init` resources which we can
 use in combination with our list of `aws_instance.web` resources.
+
+## Math
+
+Simple math can be performed in interpolations:
+
+```
+variable "count" {
+  default = 2
+}
+
+resource "aws_instance" "web" {
+  // ...
+  count = "${var.count}"
+
+  // tag the instance with a counter starting at 1, ie. web-001
+  tags {
+    Name = "${format("web-%03d", count.index + 1)}"
+  }
+}
+```
+
+The supported operations are:
+
+- *Add*, *Subtract*, *Multiply*, and *Divide* for **float** types
+- *Add*, *Subtract*, *Multiply*, *Divide*, and *Modulo* for **integer** types
+
+-> **Note:** Since Terraform allows hyphens in resource and variable names,
+it's best to use spaces between math operators to prevent confusion or unexpected
+behavior. For example, `${var.instance-count - 1}` will subtract **1** from the
+`instance-count` variable value, while `${var.instance-count-1}` will interpolate
+the `instance-count-1` variable value.