[lib] always use sub-namespace in itemdecls

[zygoloid]

For declarations in sub-namespaces, particularly within std::ranges, our itemdecls are often very unclear about which declaration they're referring to. For example, the only clue that the advance described in [iterator.operations] is in a different namespace from the advance described in [range.iterator.operations.advance] is the subclause name and how it's cross-referenced from the synopsis. That's not good enough.

We should consistently use a ranges:: prefix on the declarator-id for entities in namespace std::ranges (and likewise for other sub-namespaces of std).

[JohelEGP]

Should apply to some algorithms in utilities.tex within ranges::, like uninitialized_copy.