luci-app-statistics: Add help text to RRD time spans

Add clarifying help text to the RRD time span definition.

Only a few strings are allowed and it is easy to cause uninteded
results especially with minutes as only "min" is for minute while
("m" or "month") are for month.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>