What are the common documentation standards for most companies that primarily use C++?

I'd like to publish a library as open source.

There isn't really one thing that "most companies" do

Some do too much - see e.g. comments in Bloomberg https://github.com/bloomberg/bde/blob/master/groups/bdl/bdlma/bdlma_buffermanager.h (or any other file in that repo)

Some do nothing - see e.g. comments in Microsoft's Terminal https://github.com/microsoft/terminal/blob/main/src/server/WaitQueue.h

I suppose a lot do use doxygen or compatible notation - see .e.g Morgan Stanley https://github.com/morganstanley/binlog/blob/main/include/binlog/Range.hpp

Either way, recruiters won't care, but programmers who glance at it before interviewing you might say "oh cool, this one at least heard of doxygen"

Generating documentation from code annotations is the way to go, I think.

Doxygen is the "de facto" standard for generating documentation from C++ code annotations:
https://doxygen.nl/

Works pretty much like Javadoc in the Java world.

Cubbi, kigar64551, thank you both. I will go with doxygen.