`<gsl/span>` is intended to mirror `<span>` in the STL. `<span_ext>` provides convenience features that aren't part of `<span>`. Address #1095
The `#i13-do-not-pass-an-array-as-a-single-pointer` anchor seems to be auto generated based on the title of the section. The `#Ri-array` anchor is explicitely written in the source, so this should be a better link.
Add documentation for #1071.
