DOC: Documented new parameter for IRR function

Documented addition of IRR selection logic function. The new function contains default logic
for selection of IRR Values. User may enter their own function with custom logic if required.

numpy_financial/_financial.py

@@ -708,8 +708,9 @@ def rate(
             rn[~close] = np.nan
     return rn
 
+
 # default selection logic for IRR function when there are > 2 real solutions
-def irr_default_selection(eirr):
+def _irr_default_selection(eirr):
      # check sign of all IRR solutions
     same_sign = np.all(eirr > 0) if eirr[0] > 0 else np.all(eirr < 0)
 
@@ -727,7 +728,8 @@ def irr_default_selection(eirr):
     abs_eirr = np.abs(eirr)
     return eirr[np.argmin(abs_eirr)]
 
-def irr(values, *, raise_exceptions=False, selection_logic=irr_default_selection):
+
+def irr(values, *, raise_exceptions=False, selection_logic=_irr_default_selection):
     r"""Return the Internal Rate of Return (IRR).
 
     This is the "average" periodically compounded rate of return
@@ -749,6 +751,11 @@ def irr(values, *, raise_exceptions=False, selection_logic=irr_default_selection
         having reached the maximum number of iterations (IterationsExceededException).
         Set to False as default, thus returning NaNs in the two previous
         cases.
+    selection_logic: function, optional
+        Function for selection logic when more than 1 real solutions is found. User may 
+        insert their own customised function for selection of IRR values. 
+        The function should accept a one-dimensional array of numbers and return a number.
+        
 
     Returns
     -------