Documented the permitted syntax for template variable names.

Variables look like this: ``{{ variable }}``. When the template engine

encounters a variable, it evaluates that variable and replaces it with the

result.

result. Variable names consist of any combination of alphanumeric characters

and the underscore (``"_"``). The dot (``"."``) also appears in variable

sections, although that has a special meaning, as indicated below.

Importantly, *you cannot have spaces or punctuation characters in variable

names.*

Use a dot (``.``) to access attributes of a variable.