CREATE USER is an alias for CREATE ROLE, not its own command any longer,
so clean up references to the 'sql-createuser' link to go to
'sql-createrole' instead.

In passing, change a few cases of 'CREATE USER' to be
'CREATE ROLE ...  LOGIN'.  The remaining cases appear reasonable and
also mention the distinction between 'CREATE ROLE' and 'CREATE USER'.
Also, don't say CREATE USER "assumes" LOGIN, but rather "includes".

Patch-by: David G. Johnston, with assumes->includes by me.
Discussion: https://postgr.es/m/CAKFQuwYrbhKV8hH4TEABrDRBwf=gKremF=mLPQ6X2yGqxgFpYA@mail.gmail.com