Where is the `--` (double dash) argument documented?

This is a POSIX requirement for all utilities, see POSIX chapter 12.02, Guideline 10 for more information:

The first -- argument that is not an option-argument should be accepted as a delimiter indicating the end of options. Any following arguments should be treated as operands, even if they begin with the '-' character.

POSIX recommends all utilities to follow these guidelines.

There are a few exceptions like echo (read at OPTIONS). And special builtins that do not follow the guidelines (like break, dot, exec, etc.) :

Some of the special built-ins are described as conforming to XBD Utility Syntax Guidelines. For those that are not, the requirement in Utility Description Defaults that "--" be recognized as a first argument to be discarded does not apply and a conforming application shall not use that argument.

The intent is to document all commands that do not follow the guidelines in their POSIX man page, from POSIX chapter 12.02 third paragraph:

Some of the standard utilities do not conform to all of these guidelines; in those cases, the OPTIONS sections describe the deviations.

As the cat POSIX man page documents no deviations in the OPTIONS section, it is expected that it accept -- as a valid argument.

There still may be (faulty) implementations that fail to follow the guideline.

In particular, most GNU core utilities follow this guideline

For Linux OS that have a GNU userland: Debian, RedHat, Ubuntu, Mint, etc. (not some BSD's) the GNU common practices are important:

The list of utilities in your question have in common that they are GNU, in fact they all are GNU coreutils. The full GNU coreutils list is longer.

Most of the utilities from GNU share a common user interface and a common set of basic command line options like --help, --version and the use of -- as the signal for end of options.

The entry at info common and the GNU web page contain:

Certain options are available in all of these programs. Rather than writing identical descriptions for each of the programs, they are described here. (In fact, every GNU program accepts (or should accept) these options.)

Delimit the option list. Later arguments, if any, are treated as operands even if they begin with ‘-’. For example, ‘sort -- -r’ reads from the file named -r.

A more precise (but not that easy to explain and remember) command to get to the -- entry is (please copy exactly) info -- cat --.

It is also documented in getopt(1) and getopt(3).

For example,

Each parameter after a `--' parameter is always interpreted as a non-option parameter.