Remove quote marks around "Structural Element" once defined

Once the term _Structural Element_ has been defined, then there's no need to include it within quotes. The title case already indicates it has a special meaning in the context of this document. Before this patch, and ignoring plural variations, it appear as:

 * "Structural Element"
 * 'Structural Element'
 * "Structural element"
 * Structural Element

This patch converts them all to the final format (title case without quotes), except for the occurrence in the definition, and the occurrence in the definition of a previous term.

If accepted, this could also be done with instances of "Tag", "Inline PHPDoc", "DocComment", "DocBlock" etc - once you get past the Definitions section, there shouldn't be many occurrences of `"`.