meson: Improve certifi documentation on macOS

meson: Improve certifi documentation on macOS

First, just installing certifi doesn't install the ca-cert in the
right location. The `Install Certificates.command` script also
symlinks the openssl cert.pem to the certifi ca cert file

Second, we can make it more likely that users will notice this if we
make it a warning. If we ever get a bug report about this despite
these measures, we can try to make this an error.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2707>