Git excludes fails to avoid git submodule warning

Working with Git 2.21.0 on Linux:

The git-add(1) man page says:

"The optional configuration variable core.excludesFile indicates a path to a file containing patterns of file names to exclude from git-add"

But if I do this:

$ echo "docs/themes/*/.git >> .gitexcludes
$ git config core.excludeFiles .gitexcludes
$ git add docs
 warning: adding embedded git repository: docs/themes/foo

It seems to me that this goes against what the git-add(1) man page says.

A workaround I found on stackoverflow is to add any file within the target directory:

$ git add docs/themes/foo/README

And then add everything:

$ git add docs

In which case the contents of .gitexcludes is honoured. But that seems like a hack.

Am I experiencing the intended result of Git's exclude function? If so, I'll happily submit a patch to the man page to clarify this behaviour. If not, I guess I'm submitting a bug report here.

Thanks for all the amazing work.

Cheers.