-is unset, or if the history file is unwritable, the history is
-not saved. After saving the history, the history file is truncated
-to contain no more than @env{$HISTFILESIZE}
-lines. If @env{HISTFILESIZE} is not set, no truncation is performed.
+is unset, or if the history file is unwritable, the history is not saved.
+After saving the history, the history file is truncated
+to contain no more than @env{$HISTFILESIZE} lines.
+If @env{HISTFILESIZE} is unset, or set to null, a non-numeric value, or
+a numeric value less than zero, the history file is not truncated.