TST: add concrete examples of dataframe fixtures to docstrings

[simonjayhawkins]

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]

[codecov-io]

Codecov Report

Merging #26593 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #26593      +/-   ##
==========================================
- Coverage   91.84%   91.84%   -0.01%     
==========================================
  Files         174      174              
  Lines       50644    50644              
==========================================
- Hits        46516    46512       -4     
- Misses       4128     4132       +4

Flag	Coverage Δ
#multiple	90.37% <ø> (ø)	⬆️
#single	41.68% <ø> (-0.12%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	97% <0%> (-0.12%)	⬇️

---

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[codecov-io]

Codecov Report

Merging #26593 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #26593      +/-   ##
==========================================
- Coverage   91.84%   91.84%   -0.01%     
==========================================
  Files         174      174              
  Lines       50644    50644              
==========================================
- Hits        46516    46512       -4     
- Misses       4128     4132       +4

Flag	Coverage Δ
#multiple	90.37% <ø> (ø)	⬆️
#single	41.68% <ø> (-0.12%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	97% <0%> (-0.12%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7f31865...f109c21. Read the comment docs.

[jreback]

thanks @simonjayhawkins