TST: add concrete examples of dataframe fixtures to docstrings
add concrete examples of dataframe fixtures to docstrings to make it easier when looking for fixtures to reuse.

and also appear when using `pytest --fixtures` and `pytest --fixtures-per-test` so could be an aid to debugging

```
------------------ fixtures used by test_set_index_directly -------------------
----------------- (pandas\tests\frame\test_alter_axes.py:20) ------------------
add_imports
    no docstring available
configure_tests
    no docstring available
doctest_namespace
    Fixture that returns a :py:class:`dict` that will be injected into the namespace of doctests.
float_string_frame
    Fixture for DataFrame of floats and strings with index of unique strings

    Columns are ['A', 'B', 'C', 'D', 'foo'].

                       A         B         C         D  foo
    w3orJvq07g -1.594062 -1.084273 -1.252457  0.356460  bar
    PeukuVdmz2  0.109855 -0.955086 -0.809485  0.409747  bar
    ahp2KvwiM8 -1.533729 -0.142519 -0.154666  1.302623  bar
    3WSJ7BUCGd  2.484964  0.213829  0.034778 -2.327831  bar
    khdAmufk0U -0.193480 -0.743518 -0.077987  0.153646  bar
    LE2DZiFlrE -0.193566 -1.343194 -0.107321  0.959978  bar
    HJXSJhVn7b  0.142590  1.257603 -0.659409 -0.223844  bar
    ...              ...       ...       ...       ...  ...
    9a1Vypttgw -1.316394  1.601354  0.173596  1.213196  bar
    h5d1gVFbEy  0.609475  1.106738 -0.155271  0.294630  bar
    mK9LsTQG92  1.303613  0.857040 -1.019153  0.369468  bar
    oOLksd9gKH  0.558219 -0.134491 -0.289869 -0.951033  bar
    9jgoOjKyHg  0.058270 -0.496110 -0.413212 -0.852659  bar
    jZLDHclHAO  0.096298  1.267510  0.549206 -0.005235  bar
    lR0nxDp1C2 -2.119350 -0.794384  0.544118  0.145849  bar

    [30 rows x 5 columns]

```
