GH17483 More 'infer' keyword documentation

Author: cmazzullo

doc/source/io.rst

@@ -103,15 +103,19 @@ Column and Index Locations and Names
 ++++++++++++++++++++++++++++++++++++
 
 header : int or list of ints, default ``'infer'``
-  Row number(s) to use as the column names, and the start of the data. Default
-  behavior is as if ``header=0`` if no ``names`` passed, otherwise as if
-  ``header=None``. Explicitly pass ``header=0`` to be able to replace existing
-  names. The header can be a list of ints that specify row locations for a
-  multi-index on the columns e.g. ``[0,1,3]``. Intervening rows that are not
-  specified will be skipped (e.g. 2 in this example is skipped). Note that
-  this parameter ignores commented lines and empty lines if
-  ``skip_blank_lines=True``, so header=0 denotes the first line of data
-  rather than the first line of the file.
+  Row number(s) to use as the column names, and the start of the
+  data. Default behavior is to infer the column names: if no names are
+  passed the behavior is identical to ``header=0`` and column names
+  are inferred from the first line of the file, if column names are
+  passed explicitly then the behavior is identical to
+  ``header=None``. Explicitly pass ``header=0`` to be able to replace
+  existing names. The header can be a list of ints that specify row
+  locations for a multi-index on the columns
+  e.g. ``[0,1,3]``. Intervening rows that are not specified will be
+  skipped (e.g. 2 in this example is skipped). Note that this
+  parameter ignores commented lines and empty lines if
+  ``skip_blank_lines=True``, so header=0 denotes the first line of
+  data rather than the first line of the file.
 names : array-like, default ``None``
   List of column names to use. If file contains no header row, then you should
   explicitly pass ``header=None``. Duplicates in this list will cause