Under template instantiation, Symbol is included as an alternative under TemplateArgument. The syntax of symbol isn't described anywhere, though. Also, the text below doesn't describe what instantiation looks like with a symbol argument. I get the impression that Symbol is mistakenly located here rather than under TemplateParameter in the section on template declarations.
The grammar is given here: dlang.org/template.html#Symbol And it's described under TemplateAliasParameter.