Minor editorial changes

[jpullmann]

As suuggested by @andrea-perego following updates were made:

• Rephrased UC26 as of issue UCR: Re-phrase description of UC26 #700
• Spec references were enclosed into an "informative" context as of issue UCR: Editorial issues #683

[aisaac]

Thanks @jpullmann .
I'm assuming that this fixes the issue of undesired references without waiting for a possible change in reSpec. Maybe it's not the ideal solution, but it seems actually alright.
On the minor changes, there's now a problem with a sentence "This information is Lars G. Svensson and Phil Archer.normally"

[andrea-perego]

I made a commit addressing @aisaac 's comment, plus a minor fix.

BTW, I raised the issue about normative/informative refs in ReSpec, and a fix in under-way - see https://github.com/w3c/respec/issues/1754#issuecomment-458749170

After that fix will be implemented, we won't need class="informative" any longer.

[jpullmann]

thanks @aisaac for the issue report and @andrea-perego for handling!