Add documentation examples for formatValue()

Add example showing how to forward format specifiers.
Add example highlighting the similarity with formattedWrite(), and
comparing their use.
I just tested and if I omit $(D formatValue), the function name is still displayed in green but not in boldface, while $(D formattedWrite) still appears in black and boldface.
I also replaced /** */ with /++ +/. I will update in a few minutes.
BTW, I am not quite used to the GitHub workflow. Is it better to merge all the commits (rebase -i), amend the first one (probably not) to keep the history clean or just leave them all ?