Update doc for pd.MultiIndex.droplevel to reflect inplace behaviour (#50521)

Update documentation for pd.MultiIndex.droplevel to reflect inplace behaviour

Author: LakshyAAAgrawal

pandas/core/indexes/base.py

@@ -1963,7 +1963,7 @@ def droplevel(self, level: IndexLabel = 0):
         Return index with requested level(s) removed.
 
         If resulting index has only 1 level left, the result will be
-        of Index type, not MultiIndex.
+        of Index type, not MultiIndex. The original index is not modified inplace.
 
         Parameters
         ----------