_s: Provide a context for the comments title strings.

There are languages that have more than one plural form. Sometimes these
languages need to add the %1$s placeholder in the original singular
version to help differentiate between these plural forms.

In case the original singular version ('One thought on
&ldquo;%2$s&rdquo;' in this case) is used elsewhere, the translation
with the faulty %1$s will be used, possibly ending up with the
placeholder not being replaced.

The context will prevent that from happening by seperating the strings.