Review and update GLFM doc: features extended from standard Markdown
Problem to solve
Features extended from standard Markdown is not really correct - for example, blockquotes are a part of standard markdown - multiline blockquotes is our extension of it. In "Code spans and blocks" section, normal inline code and blocks using backticks is standard, the syntax highlighting is our extension.
I'll work with @digitalmoksha to review the GLFM doc for inaccuracies, focusing on the part mentioned above.