Merge 6.7.4 and 6.7.5

[frivoal]

6.7.5 allows feature additions to extensible RECs, which 6.7.4 does not allow to regular RECs. Other than that, the two are largely identical, and duplicate a lot of text. This should be refactored into a single section.

The difference between the two should still be called out, but this much text duplication is not needed.

Note: this is probably easier to do after #340.

[frivoal]

Done in 757c532

[frivoal]

While not completely merged, much redundancy has been removed by the commit mentioned above. Closing.