Clarify NULL in STDDEV and VARIANCE functions [CR 470567]

In the “SQL Functions” chapter, the STDDEV and VARIANCE functions contain an incorrect parameter, NULL. The information on NULL has been moved to the Usage section and is reworded for clarity:

NULL Returns NULL for a one-element input set in Sybase IQ 12.7. In prior versions, NULL returned zero.

This correction was also made to STDDEV_SAMP() and VAR_SAMP() functions and will appear in the next release of the Sybase IQ documentation.