Bold and italic follow Asciidoc semantics - #408

We only highlight words. So this PR doesn't really make sense.

The book uses ** for highlighting whole words:

The first option ignores whitespace **completely** when comparing lines [...]

The Asciidoc documentation (my emphasis):

To render a word or phrase with bold styling, place an asterisk (*) at the beginning and end of the text you wish to format. To bold a letter or letters in a string of text, place two asterisks (**) before and after the letter or letters.

As their example clearly states, ** is used only for highlighting letters as parts of a word:

*charm*.
**u**ltimate

As you said, this book highlights only words and phrases, so it should use * instead of **.

I assume that the ** syntax this book uses is a leftover from the first edition when the book was written using Markdown because using * in Markdown renders italic and ** renders bold. In Asciidoc, * is used for bold words and phrases and _ is used for italic words and phrases.