Update fromxls doc string
Clarify what can be used for sheet; it is not immediately obvious without reading the code.
@@ -13,6 +13,8 @@ def fromxls(filename, sheet=None, use_view=True):
"""
Extract a table from a sheet in an Excel .xls file.
+ Sheet is identified by its name or index number.
+
N.B., the sheet name is case sensitive.
"""
