[IMP] core: clarify docs about config --test-tags

Technical name is not added to test_tags since https://github.com/odoo/odoo/commit/95b4f2ab4b5698ab3a28c9c35ac8da6fb6def983

at_install tag is added by default since introducing @tagged decorator: https://github.com/odoo/odoo/commit/b356b190338e3ee032b9e3a7f670f76468965006



Clarify how special tags at_install/post_install work.

Also, add dots for @tagged doc, because otherwise we have a mess in sphinx docs.

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task-2431630

closes odoo/odoo#71130

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>