config: Document popular blackfriday options (#316)

exampleSite/config.toml

@@ -21,8 +21,12 @@ defaultContentLanguageInSubdir = false
 [outputs]
   home = [ "HTML", "CSS" ]
 
+# Configure BlackFriday Markdown rendering.
+#   See: https://gohugo.io/readfiles/bfconfig/
 [blackfriday]
-  hrefTargetBlank = true
+  hrefTargetBlank = true  # `true` opens external links in a new tab.
+  fractions = true  # `false` disables smart fractions (e.g. 5/12 formatted as a fraction).
+  smartypants = true  # `false` disables all smart punctuation substitutions (e.g. smart quotes, dashes, fractions).
 
 [params]
   # Color theme.