bug#36827: Say "if non-nil" or "if t" consistently

[積丹尼 Dan Jacobson]

    kill-do-not-save-duplicates is a variable defined in ‘simple.el’.
    Its value is nil

    Documentation:
    Do not add a new string to ‘kill-ring’ if it duplicates the last one.


OK, but do like

    add-log-always-start-new-record is a variable defined in ‘add-log.el’.
    Its value is nil

    Documentation:
    If non-nil, ‘add-change-log-entry’ will always start a new record.

i.e., say

    If non-nil do not add a new string to ‘kill-ring’ if it duplicates the last 
one.
    ^^^^^^^^^^

(if that is indeed the case.)