DOC: Broaden Exact Test Reference

* Broadens reference to exact tests.
* Adds Barnard's Exact as a less conservative, unconditional alternative to Fisher's Exact.

Co-authored-by: Tirth Patel <tirthasheshpatel@gmail.com>
Co-authored-by: Daniel Scott <info@synapseproduction.com>
Co-authored-by: Pamphile ROY <roy.pamphile@gmail.com>

scipy/stats/stats.py

@@ -6663,18 +6663,18 @@ def chisquare(f_obs, f_exp=None, ddof=0, axis=0):
     See Also
     --------
     scipy.stats.power_divergence
-    scipy.stats.fisher_exact : A more powerful alternative to the chisquare
-                               test if any of the frequencies are less than 5.
+    scipy.stats.fisher_exact : Fisher exact test on a 2x2 contingency table.
+    scipy.stats.barnard_exact : An unconditional exact test. An alternative
+        to chi-squared test for small sample sizes.
 
     Notes
     -----
     This test is invalid when the observed or expected frequencies in each
     category are too small.  A typical rule is that all of the observed
-    and expected frequencies should be at least 5. If one or more frequencies
-    are less than 5, Fisher's Exact Test can be used with greater statistical
-    power. According to [3]_, the total number of samples is recommended to be
-    greater than 13, otherwise a table-based method of obtaining p-values is 
-    recommended.
+    and expected frequencies should be at least 5. According to [3]_, the
+    total number of samples is recommended to be greater than 13,
+    otherwise exact tests (such as Barnard's Exact test) should be used
+    because they do not overreject.
 
     Also, the sum of the observed and expected frequencies must be the same
     for the test to be valid; `chisquare` raises an error if the sums do not
@@ -6696,7 +6696,7 @@ def chisquare(f_obs, f_exp=None, ddof=0, axis=0):
     .. [2] "Chi-squared test", https://en.wikipedia.org/wiki/Chi-squared_test
     .. [3] Pearson, Karl. "On the criterion that a given system of deviations from the probable 
            in the case of a correlated system of variables is such that it can be reasonably 
-           supposed to have arisen from random sampling", Philosophical Magazine. Series 5. 50
+           supposed to have arisen from random sampling", Philosophical Magazine. Series 5. 50 
            (1900), pp. 157-175.
 
     Examples