update docstring robust quantile

Co-authored-by: Michael Deceglie <[email protected]>

rdtools/utilities.py

@@ -6,7 +6,7 @@ def robust_quantile(x, q):
     Compute the q-th quantile of a time series (x), ignoring small values and NaN's.
     NaN's and small values [x < Q(x,q)/1000] are removed before calculating the quantile.
     This function ensures that time series with NaN's and distributions without
-    NaN's return the same results.
+    NaN's return the same results. Should only be used if x is expected to be ≥0.
 
     Parameters
     ----------