Add new sections regarding schemas and reflection

Add new sections regarding schemas and reflection

* add a new section to reflection.rst `Schemas and Reflection`.
  * this contains some text from the ticket
* migrate some text from `Specifying the Schema Name` to new section
* migrate some text from PostgreSQL dialect to new section
  * target text is made more generic
  * cross-reference the postgres and new sections to one another, to avoid duplication of docs
* update some docs 'meta' to 'metadata_obj'

Fixes: #4387
Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Change-Id: I2b08672753fb2575d30ada07ead77587468fdade
(cherry picked from commit 0fa0beacb465c61e792c97d530a0e8fdd7139256)