ISQL changes for Firebird 6.0

[mrotteveel]

For ISQL, we need to document the following changes for Firebird 6.0

• EXPLAIN command (see Feature #7675 - EXPLAIN statement and RDB$SQL package. firebird#7697, and doc/README.isql_enhancements)
• SET AUTOTERM ON/OFF command (see Feature SET AUTOTERM in ISQL firebird#7868 and doc/README.isql_enhancements)

(We're not modifying ISQL just yet, as it covers multiple versions, and I prefer to only cover (nearly) released versions.)