Doc: clarify introduction to database roles.

Doc: clarify introduction to database roles.

Word-smith section 22.1 ("Database Roles") a little bit in hopes
of removing confusion about how the bootstrap superuser's name
is chosen.

While here, I couldn't help noticing that the claim that the bootstrap
superuser is the only initially-existing role has been a lie since
we started to invent predefined roles.  We don't want too much detail
in this very introductory text, but it seems worth changing it to say
that it's the only initially-existing login-capable role.

Per documentation comment from Maja Zaloznik.

Discussion: https://postgr.es/m/167931662853.3349090.18217722739345182859@wrigleys.postgresql.org