Documentation improvements for Sequence objects

Documentation tweaks needed for Sequence Objects

1. Users may experience confusion between SEQUENCE Storage Engine and SEQUENCE Object.
https://mariadb.com/kb/en/mariadb/create-sequence/
https://mariadb.com/kb/en/mariadb/sequence/

Both chapters should include small disclaimer which clarifies that these are unrelated syntax elements, despite having similar name.

2. Sequence objects act as regular tables in many contexts, including LOCK TABLES command.
This may complicate migration from other RDBMS which use both LOCK TABLES and Sequence objects, because e.g. in Oracle LOCK TABLES does allow to increment sequences. This difference should be documented.