Changes

Please provide a brief description of the changes here.

In the spec compliance matrix, add header lines for readability.

This fixes two issues:

• first, having an entry as blank forces the reader to consider whether this is a missing feature in a SIG implementation, which is an issue, or weather this is a header line, in which case a blank is not an issue.
• second, when many rows are present, reading the spec matrix in a web browser (or in an editor) is difficult because the column names are no longer rendered on the screen when scrolling down to see more content.

Repeating the header column names for intermediate sections fixes both issues.

For non-trivial changes, follow the change proposal process.

• Related issues #
• Related OTEP(s) #
• Links to the prototypes (when adding or changing features)
• CHANGELOG.md file updated for non-trivial changes
• spec-compliance-matrix.md updated if necessary