The documentation for ---default-new-key-algo says that the default is
rsa2048/cert,sign+rsa2048/encr but
gpg-default-to-3072-bit-RSA-keys.patch changes this to
rsa3072/cert,sign+rsa3072/encr.
I suggest updating the wording so:
This option can be used to change the default algorithms for key
generation. The STRING is similar to the arguments required for
the command '--quick-add-key' but slightly different. For example
the current default of '"rsa3072/cert,sign+rsa3072/encr"' can be changed
to the value of what we currently call future default, which is
'"ed25519/cert,sign+cv25519/encr"'. You need to consult the source code
to learn the details. Note that the advanced key generation commands
can always be used to specify a key algorithm directly.
The default of rsa3072 is specific to Debian. Upstream GPG still
defaults to rsa2048.