[implimits] Add cross-references for Annex B

[jensmaurer]

For the bullets in Annex B, add cross-references to where the features are defined.

[jensmaurer]

Do we want the cross-reference

• as the first item in the bullet, without parentheses
• before the quantity bracket, in parentheses
• after the quantity bracket, in parentheses
• somewhere else?

[jensmaurer]

This section needs further work, e.g.

p2.20: "Data members in a single class (12.2) [16 384]." vs. p2.28: "Members declared in a single class (12.2) [4 096]." Do we mean non-static data members? What about inherited members?

p2.40 "Template arguments in a template declaration (17.1) [1 024].": I thought these are "template parameters"?

p2.37 "Nested external specifications [1 024].": What are these? Linkage specifications [dcl.link]?

[jensmaurer]

Drive-by finding are covered by #1794.