Add example for bold-italic text

[Richard Cotton]

For section Span Elements -> Emphasis.

To make a block of text bold and italic, you can use three asterisks, three underscores, or two of one and one of the other.

three asterisks
three underscores
two asterisks and one underscore

***three asterisks***
___three underscores___
**_two asterisks and one underscore_**

If you want to nest italic within bold (or the other way around), then you must mix asterisks and underscores.

two asterisks and one underscore
one asterisk and two underscores

**two asterisks and _one underscore_**
*one asterisk and __two underscores__*

Using three asterisks or underscores does not work.

three asterisks
three underscores

**three *asterisks***  
__three _underscores___