P0009: mdspan declaration still has span(); superfluous section [mdspan.basic.codomain]

1. P0009's changelog claims that mdspan's span() method was removed in R11. However, the method still exists in mdspan's class declaration, even though there is no description for span().
2. The **[mdspan.basic.codomain]** ("mdspan observers of the codomain") section exists only as a comment in mdspan's class declaration. It is not used as an actual section name anywhere in the document. The section should be superfluous, given that nothing in the class document beyond that point requires description. (All the methods after span() are defined inline, and the class' members are exposition only.)

Inspect P0009's changelog and the mdspan class declaration, focusing on the span() declaration and the [mdspan.basic.codomain] comment. Done means the declaration and superfluous section comment are consistent with the documented R11 changes, with no remaining references requiring that section.