Rename some doc files and commands related to importing

Since we'll likely have generic SQLite database importing
via [pgloader](https://pgloader.io/) for migrating bridge
databases from SQLite to Postgres, we'd rather avoid
calling the "import Synapse SQLite database" command
as just `--tags=import-sqlite-db`.

Similarly, for the media store, we'd like to mention that it's
related to Synapse as well.

We'd like to be more explicit, so as to be less confusing,
especially in light of other homeserver implementations
coming in the future.

roles/matrix-postgres/tasks/main.yml

@@ -19,10 +19,12 @@
   tags:
     - import-postgres
 
-- import_tasks: "{{ role_path }}/tasks/import_sqlite_db.yml"
+# The `run_postgres_import_sqlite_db` variable had better be renamed to be consistent,
+# but that's a breaking change which may cause trouble for people.
+- import_tasks: "{{ role_path }}/tasks/import_synapse_sqlite_db.yml"
   when: run_postgres_import_sqlite_db|bool
   tags:
-    - import-sqlite-db
+    - import-synapse-sqlite-db
 
 - import_tasks: "{{ role_path }}/tasks/upgrade_postgres.yml"
   when: run_postgres_upgrade|bool