SQL: add release notes for refactor (GH6292)
DOC: update sql docs

- minor edits
- remove mention of object PandasSQLWithEngine, as this API is not yet finalized yet, so not yet meant for public use
SQL: update some signatures and docstrings:

- remove meta kwarg from read_sql_table (see discussion in #6300)
- remove flavor kwarg from read_sql (not necessary + not there in 0.13, so would have been API change)
- update docstring of to_sql in generic with latest changes
- enhance docstring of get_schema
Merge pull request #7120 from jorisvandenbossche/sql-release

SQL: add release notes for refactor (GH6292)