The usage of DEB_BUILD_PROFILES was not documented in the man page.
This change does exactly that in a similar way other tools describe
the DEB_BUILD_PROFILES variables (see e.g. dpkg-buildpackage).
I'll contribute another PR later to add a -P option (like the one dpkg-buildpackage has), which
will override the DEB_DEBUILD_PROFILES environment variable.
I think that helps making those tools more intuitive ;)