DOC: mention syntax for using parameters in read_sql in Notes in docstring

[jorisvandenbossche]

See http://stackoverflow.com/questions/24408557/pandas-read-sql-with-parameters.

Add a note mentioning this is driver-dependent which works, and they have to look at their drivers documentation.

[jnmclarty]

Done. Added same comment to read_query as well. Submitted PR #8266