Change is partly a verbatim copy of https://www.kernel.org/doc/Documentation/ABI/stable/vdso
Quote: "Programs that dynamically link to glibc will use the vDSO automatically. Otherwise, you can use the reference parser in tools/testing/selftests/vDSO/parse_vdso.c."
The old version of the book was slightly imprecise. It was unclear to me whether this is an optional or a default behavior.
There are multiple different system calls. Starting the introduction in plural gives a better bridge to the examples "like to read or to write to a file, to open a socket" ...
Cornelius Diekmann