update docs

pyam/core.py

@@ -1509,16 +1509,16 @@ def compare(left, right, left_label='left', right_label='right',
 
 
 def concat(dfs):
-    """Concatenate a series of dataframes together"""
+    """Concatenate a series of `pyam.IamDataFrame`-like objects together"""
     if not hasattr(dfs, '__iter__'):
-        raise TypeError('Input data must be iterable')
-
-    df = None
-    for _df in dfs:
-        if not isinstance(_df, IamDataFrame):
-            raise TypeError('Input contains non-dataframe')
-        if df:
-            df.append(_df, inplace=True)
+        raise TypeError('Input data must be iterable (e.g., list or tuple)')
+
+    _df = None
+    for df in dfs:
+        if not isinstance(df, IamDataFrame):
+            raise TypeError('Input contains non-`pyam.IamDataFrame`')
+        if _df is None:
+            _df = copy.deepcopy(df)
         else:
-            df = copy.deepcopy(_df)
-    return df
+            _df.append(df, inplace=True)
+    return _df