DOC: Improve quantile docstring (#56457)

DOC: improve quantile docstring

Author: linus-md

pandas/core/series.py

@@ -2820,8 +2820,8 @@ def quantile(
             This optional parameter specifies the interpolation method to use,
             when the desired quantile lies between two data points `i` and `j`:
 
-                * linear: `i + (j - i) * fraction`, where `fraction` is the
-                  fractional part of the index surrounded by `i` and `j`.
+                * linear: `i + (j - i) * (x-i)/(j-i)`, where `(x-i)/(j-i)` is
+                  the fractional part of the index surrounded by `i > j`.
                 * lower: `i`.
                 * higher: `j`.
                 * nearest: `i` or `j` whichever is nearest.