Made some changes to the Packer templating docs

1. Added the `user` function.
2. Some tidying up of language.

website/source/docs/templates/engine.html.md

@@ -18,13 +18,12 @@ For example, the `{{timestamp}}` function can be used in any string to generate
 the current timestamp. This is useful for configurations that require unique
 keys, such as AMI names. By setting the AMI name to something like
 `My Packer AMI {{timestamp}}`, the AMI name will be unique down to the second.
-If you need greater than one second granularity, you should use `{{uuid}}`, as
-when you have multiple builders in the same template.
+If you need greater than one second granularity, you should use `{{uuid}}`, for example when you have multiple builders in the same template.
 
-In addition to globally available functions like timestamp shown before, some
-configurations have special local variables that are available only for that
-configuration. These are recognizable because they're prefixed by a period, such
-as `{{.Name}}`.
+In addition to globally available functions like `{{timestamp}}`, some
+configurations have special local variables that are available only for
+that configuration. These are recognizable because they're prefixed by a
+period, such as `{{.Name}}`.
 
 The complete syntax is covered in the next section, followed by a reference of
 globally available functions.
@@ -33,9 +32,9 @@ globally available functions.
 
 The syntax of templates uses the following conventions:
 
-* Anything template related happens within double-braces: `{{ }}`
-* Variables are prefixed with a period and capitalized, such as `{{.Variable}}`
-* Functions are directly within the braces, such as `{{timestamp}}`
+* Anything template related happens within double-braces: `{{ }}`.
+* Variables are prefixed with a period and capitalized, such as `{{.Variable}}`.
+* Functions are directly within the braces, such as `{{timestamp}}`.
 
 Here is an example from the VMware VMX template that shows configuration
 templates in action:
@@ -72,6 +71,7 @@ in Packer templates. These are listed below for reference.
 - `timestamp` - The current Unix timestamp in UTC.
 - `uuid` - Returns a random UUID.
 - `upper` - Uppercases the string.
+- `user` - Specify a user variable.
 
 ### isotime Format