pre2.3.1-9: Merged Manfred's documentation updates

doc update: wherefirst_ <-> wherelast_ typo
doc update: using \NULL text macro more consistently for S-Lang's NULL literal
Author:    Manfred Hanke <Manfred.Hanke@tngtech.com>
pre2.3.1-10: Use a macro in the the case statements of the wherefirst/last_op code.