The -- mechanism in general, not specific to the cp command albeit mostly limited to programs that use getopt/popt for their argument parsing, is described all over the place. I first encountered it in a book, written by Eric Foxley, entitled UNIX for Super-Users and published in 1985.

* It's perennially asked about on Unix and Linux Stack Exchange, ranging from https://unix.stackexchange.com/q/11376/5132 almost a decade ago to https://unix.stackexchange.com/q/570729/5132 last year.

* It's in a whole bunch of duplicate questions there, including (but not limited to) https://unix.stackexchange.com/questions/linked/1519?lq=1 .

* It's in the Single Unix Specification, and has been for a long time. (https://pubs.opengroup.org/onlinepubs/7990989775/xbd/utilcon...)

* It's in the getopt(3) manual page from the Linux man-pages project. It's in the getopt(1) manual page from the util-linux project.

* It's in getopt(1) and getopt(3) in the NetBSD Manual. The same goes for the OpenBSD and FreeBSD manuals.

* It's in Blum's and Bresnehan's 2015 Linux Command Line and Shell Scripting Bible, on page 377.

* It's in Marcel Gagné's 2002 Linux System Administration: A User's Guide, on page 48.

* It's in Kirk Waingrow's 1999 UNIX Hints & Hacks.

* It was Q&A number 32 in part 2 of the Usenet comp.unix.shell FAQ document. (http://cfajohnson.com/shell/cus-faq-2.html#Q32)

Specific to the cp command, or rather to the many cp commands that you could end up using, as this is not a "bash command":

* No, the OpenBSD, FreeBSD, and NetBSD cp(1) manuals do not mention it.

* Neither does the GNU CoreUtils cp(1) manual page, as you noted. It's instead in the GNU CoreUtils doco section on "common options". The actual CoreUtils cp(1) manual doesn't mention that there even are "common options". (https://manpages.debian.org/sid/coreutils/cp.1.en.html) That's only mentioned, as an aside, in the info page. (https://gnu.org/software/coreutils/manual/html_node/cp-invoc...)

* For the cp command that is part of Rob Landley's toybox, this is covered in the introduction to the toybox(1) manual page, and there isn't really a distinct cp(1) manual page. (http://landley.net/toybox/help.html)

* No, BusyBox doesn't similarly document this for its cp command, or anywhere on busybox(1). (https://busybox.net/downloads/BusyBox.html)

* It's tangentially in several GNU CoreUtils FAQ Q&As, inasmuch as CoreUtils cp is the same as CoreUtils rm in this regard. (https://gnu.org/software/coreutils/faq/coreutils-faq.html#Ho...)

* It's tangentially in the Linux Documentation Project's Advanced Bash-Scripting Guide, again because cp is the same as rm here. (https://tldp.org/LDP/abs/html/special-chars.html#DOUBLEDASHR...)

* For an operating system where this is actually in the cp(1) manual itself, look to Solaris, and its successor Illumos, where you'll find it in the NOTES section. (https://illumos.org/man/1/cp)

* Another operating system where this is actually in the cp(1) manual itself is AIX, actually in the FLAGS table alongside all of the other flags. (https://ibm.com/support/knowledgecenter/ssw_aix_72/c_command...)

That is a far more comprehensive answer than I was expecting!

You're absolutely right about my system. There is a link to the coreutils common options in the cp info page. I missed that when I was searching through it.