Document stricter conversion rules

[bgrainger]

In Connector/NET, MySqlDataReader.GetX is lax, and will usually succeed when asked to convert a column of any MySqlDbType to the X CLR type (e.g., call GetDecimal on a MEDIUMINT UNSIGNED column, or call GetString on a FLOAT column).

MySqlConnector is much stricter and (currently) mostly only performs Int16/Int32/Int64 conversions.

If the strict conversion rules are kept (see #406) then they should be documented in "Migrating from Connector/NET".