Doc: document --precent-ndigits

Author: CarterLi

src/data/help.txt

@@ -70,6 +70,7 @@ Display options:
     --hide-cursor <?value>:      Whether to hide the cursor during the run
     --binary-prefix <value>:     Set the binary prefix to used. Must be IEC, SI or JEDEC. Default is IEC
     --percent-type <value>:      Set the percentage output type. 1 for percentage number, 2 for bar, 3 for both, 6 for bar only, 9 for colored number. Default is 1
+    --percent-ndigits <value>:   Set the number of digits to keep after the decimal point when formatting percentage numbers. Default is 0
     --bar-char-elapsed <str>:    Set the character to use in elapsed part. Default is '■'
     --bar-char-total <str>:      Set the character to use in total part. Default is '-'
     --bar-width <?num>:          Set the width of the bar, in number of characters. Default is 10