DOC: array creation API is defined twice in docs

The information in doc/source/user/c-info.how-to-extend.rst is repeated in `doc/source/reference/c-api.array.rst. The first should only reference the second, and the descriptive text should be merged.

This may be a good starter task. The cycle to fix this issue, based on the doc building documentation is to

1. create the documentation with make html
2. Note the many warnings about duplicate C object description in the two files
3. Edit the files to unify the documentation, one should be using .. c:function:: to define the functions and the other using :c:func: to reference it, perhaps with descriptive text.
4. Use firefox build/html/reference/c-api.array.html and firefox build/html/user/c-info.how-to-extend.html to view the changes

[mattip]

Did you run git submodule init && git submodule update?

[mattip]

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