DOC: clarify read_sql docstring on relation with other read_sql_ functions
We have 3 (!) functions for reading data from SQL:

`read_sql`
`read_sql_table`
`read_sql_query`

Definitely one of these can be removed.  Probably aiming at `read_sql_query`, since its functionality seems most similar to that of `read_sql`.

`read_sql_table` seems more like a convenience function in place of `SELECT * FROM <table>`, though one could argue that we could accept a table-name in `read_sql` to do the same thing.