chore(r): Update roxygen generated docs and non-api sentinel (#4635)

This PR updates `devtools::document()` with a more recent roxygen2 and
fixes an R C API issue (the unbound variable sentinel is no longer
"public", so we have to use another one).

For the CRAN release I also had to update nanoarrow (fixes gcc16
warnings), but I'll do that separately since nanoarrow is close to an
actual release.

Author: paleolimbot

r/adbcdrivermanager/src/radbc.cc

@@ -57,8 +57,8 @@ static int adbc_update_parent_child_count(SEXP xptr, int delta) {
   SEXP child_count_sym = PROTECT(Rf_install(".child_count"));
 #if defined(R_VERSION) && R_VERSION >= R_Version(4, 5, 0)
   SEXP child_count_sexp =
-      PROTECT(R_getVarEx(child_count_sym, parent_env, FALSE, R_UnboundValue));
-  if (child_count_sexp == R_UnboundValue) {
+      PROTECT(R_getVarEx(child_count_sym, parent_env, FALSE, R_MissingArg));
+  if (child_count_sexp == R_MissingArg) {
     Rf_error("Internal error: .child_count not found");
   }
 #else