The texinfo file doc/gpgv.texi describes how, with default options, the keybox "trustedkeys.kbx" is used or "trustedkeys.gpg" of the former is not found.
See paragraph in lines 62 to 69 in file doc/gpgv.texi.

However, the subsection "FILES" towards the end of this texinto file
a) does only list "trustedkeys.gpg" and not "trustedkeys.kbx";
b) does not list the preferred choice of file, but only the fall-back file.

I would suggest, that the FILES section does

1. list both filenames.
2. makes use of "${GNUPGHOME}" instead of "~/.gnupg" in order to be precise. However, this might be detrimental for readability and retained as "~/.gnupg".

A previous bug/improvement in the same area was: https://dev.gnupg.org/T4386