[1.9.x] Fixed #25936 -- Added showmigrations to migrations topic guide.

* :djadmin:`sqlmigrate`, which displays the SQL statements for a migration.

* :djadmin:`showmigrations`, which lists a project's migrations.

You should think of migrations as a version control system for your database

schema. ``makemigrations`` is responsible for packaging up your model changes

into individual migration files - analogous to commits - and ``migrate`` is