Add docs

Author: phofl

doc/source/user_guide/merging.rst

@@ -473,6 +473,18 @@ either the left or right tables, the values in the joined table will be
    p.plot([left, right], result, labels=["left", "right"], vertical=False);
    plt.close("all");
 
+.. ipython:: python
+
+   result = pd.merge(left, right, how="left_semi", on=["key1", "key2"])
+   result
+
+.. ipython:: python
+   :suppress:
+
+   @savefig merging_merge_on_key_inner.png
+   p.plot([left, right], result, labels=["left", "right"], vertical=False);
+   plt.close("all");
+
 .. ipython:: python
 
    result = pd.merge(left, right, how="cross")

pandas/core/frame.py

@@ -339,7 +339,7 @@
     * inner: use intersection of keys from both frames, similar to a SQL inner
       join; preserve the order of the left keys.
     * left_semi: Filter for rows in the left that have a match on the right;
-      preserve the order of the left keys.
+      preserve the order of the left keys, similar to SQL left semi join.
 
       .. versionadded:: 3.0
     * cross: creates the cartesian product from both frames, preserves the order