DOC: extract/extractall clarifications

This PR clarifies the new documentation for extract and extractall.
It was requested by @jreback in
#11386 (comment)

Author: Toby Dylan Hocking <tdhock5@gmail.com>

Closes #12281 from tdhock/extract-docs and squashes the following commits:

2019d1b [Toby Dylan Hocking] DOC: extract/extractall clarifications

Author: tdhock

doc/source/text.rst

@@ -196,9 +196,9 @@ DataFrame with one column per group.
 Elements that do not match return a row filled with ``NaN``. Thus, a
 Series of messy strings can be "converted" into a like-indexed Series
 or DataFrame of cleaned-up or more useful strings, without
-necessitating ``get()`` to access tuples or ``re.match`` objects.  The
-results dtype always is object, even if no match is found and the
-result only contains ``NaN``.
+necessitating ``get()`` to access tuples or ``re.match`` objects. The
+dtype of the result is always object, even if no match is found and
+the result only contains ``NaN``.
 
 Named groups like
 
@@ -275,15 +275,16 @@ Extract all matches in each subject (extractall)
 
 .. _text.extractall:
 
+.. versionadded:: 0.18.0
+
 Unlike ``extract`` (which returns only the first match),
 
 .. ipython:: python
 
    s = pd.Series(["a1a2", "b1", "c1"], ["A", "B", "C"])
    s
-   s.str.extract("[ab](?P<digit>\d)", expand=False)
-
-.. versionadded:: 0.18.0
+   two_groups = '(?P<letter>[a-z])(?P<digit>[0-9])'
+   s.str.extract(two_groups, expand=True)
 
 the ``extractall`` method returns every match. The result of
 ``extractall`` is always a ``DataFrame`` with a ``MultiIndex`` on its
@@ -292,30 +293,29 @@ indicates the order in the subject.
 
 .. ipython:: python
 
-   s.str.extractall("[ab](?P<digit>\d)")
+   s.str.extractall(two_groups)
 
 When each subject string in the Series has exactly one match,
 
 .. ipython:: python
 
    s = pd.Series(['a3', 'b3', 'c2'])
    s
-   two_groups = '(?P<letter>[a-z])(?P<digit>[0-9])'
 
 then ``extractall(pat).xs(0, level='match')`` gives the same result as
 ``extract(pat)``.
 
 .. ipython:: python
 
-   extract_result = s.str.extract(two_groups, expand=False)
+   extract_result = s.str.extract(two_groups, expand=True)
    extract_result
    extractall_result = s.str.extractall(two_groups)
    extractall_result
    extractall_result.xs(0, level="match")
 
 
 Testing for Strings that Match or Contain a Pattern
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+---------------------------------------------------
 
 You can check whether elements contain a pattern:
 
@@ -355,7 +355,7 @@ Methods like ``match``, ``contains``, ``startswith``, and ``endswith`` take
    s4.str.contains('A', na=False)
 
 Creating Indicator Variables
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+----------------------------
 
 You can extract dummy variables from string columns.
 For example if they are separated by a ``'|'``:

doc/source/whatsnew/v0.18.0.txt

@@ -157,50 +157,50 @@ Currently the default is ``expand=None`` which gives a ``FutureWarning`` and use
 
 .. ipython:: python
 
-   pd.Series(['a1', 'b2', 'c3']).str.extract('[ab](\d)', expand=False)
+   pd.Series(['a1', 'b2', 'c3']).str.extract('[ab](\d)', expand=None)
 
-Extracting a regular expression with one group returns a ``DataFrame``
-with one column if ``expand=True``.
+Extracting a regular expression with one group returns a Series if
+``expand=False``.
 
 .. ipython:: python
 
-   pd.Series(['a1', 'b2', 'c3']).str.extract('[ab](\d)', expand=True)
+   pd.Series(['a1', 'b2', 'c3']).str.extract('[ab](\d)', expand=False)
 
-It returns a Series if ``expand=False``.
+It returns a ``DataFrame`` with one column if ``expand=True``.
 
 .. ipython:: python
 
-   pd.Series(['a1', 'b2', 'c3']).str.extract('[ab](\d)', expand=False)
+   pd.Series(['a1', 'b2', 'c3']).str.extract('[ab](\d)', expand=True)
 
 Calling on an ``Index`` with a regex with exactly one capture group
-returns a ``DataFrame`` with one column if ``expand=True``,
+returns  an ``Index`` if ``expand=False``.
 
 .. ipython:: python
 
    s = pd.Series(["a1", "b2", "c3"], ["A11", "B22", "C33"])
    s
-   s.index.str.extract("(?P<letter>[a-zA-Z])", expand=True)
-
-It returns an ``Index`` if ``expand=False``.
-
-.. ipython:: python
-
    s.index.str.extract("(?P<letter>[a-zA-Z])", expand=False)
 
-Calling on an ``Index`` with a regex with more than one capture group
-returns a ``DataFrame`` if ``expand=True``.
+It returns a ``DataFrame`` with one column if ``expand=True``.
 
 .. ipython:: python
 
-   s.index.str.extract("(?P<letter>[a-zA-Z])([0-9]+)", expand=True)
+   s.index.str.extract("(?P<letter>[a-zA-Z])", expand=True)
 
-It raises ``ValueError`` if ``expand=False``.
+Calling on an ``Index`` with a regex with more than one capture group
+raises ``ValueError`` if ``expand=False``.
 
 .. code-block:: python
 
     >>> s.index.str.extract("(?P<letter>[a-zA-Z])([0-9]+)", expand=False)
     ValueError: only one regex group is supported with Index
 
+It returns a ``DataFrame`` if ``expand=True``.
+
+.. ipython:: python
+
+   s.index.str.extract("(?P<letter>[a-zA-Z])([0-9]+)", expand=True)
+
 In summary, ``extract(expand=True)`` always returns a ``DataFrame``
 with a row for every subject string, and a column for every capture
 group.