Documentation changes

[maxcrees]

• [x] Difference: vertical alignment changes from modifiers t and d apply to any corresponding cell, not just those that are row-spanned.
• [x] Spaces before a leading period on a line of table data may not be sufficient to escape it (investigate)
• [x] Add examples for class "A"
• [x] Difference: document other extensions?
• [x] Version history section