DOC: stats: Copyedit `stats.describe` docstring.
DOC: stats: Copyedit `stats.describe` docstring.

* Fix indentation in the "Parameters" and "Return Values" sections.
* Refer to `a` instead of "the data" or "the data array".
* For all return values, note that the statistics are computed along
  the given axis.
* A few other small copyedits.
* Fix a line length in the "Examples" section.

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