Help: Clarify references to <CONFIG>_POSTFIX target properties

Update wording added by commit 6e5ccabe9b (Genex: Update
$<TARGET_FILE_BASE_NAME:...>: take care of POSTFIX, 2019-04-25,
v3.15.0-rc1~175^2) to associate references to the `<CONFIG>_POSTFIX`
properties with references to the `OUTPUT_NAME` properties because
they are both used together.  Avoid association with the `PDB_NAME`
properties because they cause `<CONFIG>_POSTFIX` to be ignored.