Improve ddoc source style of template.dd

Proper indentation for paragraphs and code blocks will make ddoc source more human-readable.
typedef is deprecated in D2
fix Issue 3467 - Non-int integral template parameters not correctly propagated
fix Issue 2025 - Inconsistent rules for instantiating templates with a tuple parameter
Add note about the removing top qualifier for the deduced parameter types
Merge pull request #499 from 9rnsr/fix_template

Improve template.dd documentation