Verifying understanding of RedCarpet options, pt 8

Documented current behaviour of/defaults for
`:superscript` option.

RSpec: 486 examples, 0 failures
RuboCop: 127 files inspected, no offenses detected
Verifying understanding of RedCarpet options, pt 9

Documented current behaviour of/defaults for
`:underline` option.

RSpec: 489 examples, 0 failures
RuboCop: 127 files inspected, no offenses detected