DOC: clarify that umfpack, fftw and djbfft are not used by numpy

their presence in site.cfg is only for the benefit of third party
packages using numpy.distutils
DOC: document non-integer reduce axis argument deprecation
DOC: add ascontiguousarray and asarray_chkfinite to appropriate section

added to "Changing kind of array" with the other as* functions
Closes gh-4890
MAINT: add common source indexer files to .gitignore

ignore cscope and gnu global files
DOC: fix documented return value of tostring/tobytes

The function returns bytes not strings. This is relevant in python3.
DOC: add version added tag to reduction keepdims argument
DOC: document behavior of ma.sort(endswith=) for unmasked min/max values

see gh-4422
DOC: add some missing ma functions to reference

closes gh-4898
DOC: remove obsolete NA documentation page
MAINT: .gitignore += generated umath/loops.h

Commit 494abcf1 (ENH: speed-up minimum, maximum and xor for bool dtype)
reworked numpy/core/src/umath/loops.h to be autogenerated at build time,
but forgot to update .gitignore.

Fix it, so that the now-autogenerated loops.h do not constantly show as
untracked in git status output.
DOC: NPY_ITEM_LISTPICKLE -> NPY_LIST_PICKLE

There is no NPY_ITEM_LISTPICKLE - it was NPY_LIST_PICKLE from the
beginning - from c3551579 (Expand usage of hasobject to be a flag-like
entity keeping track of how the data-type should be used.)

The documentation in the same rst file even uses the correct identifier
later, so it was just a typo/thinko.
DOC: fix PEP links in HOWTO_DOCUMENT.rst.txt
DOC: add a note on the potential string dtype change
TST: test reduction argument parsing

In case we might want it up in future
DOC: numpy.testing documentation clarification of "Raise an assertion"

"Raise an assertion" was changed to "Raises an AssertionError" to clarify that
an exception named AssertionError will be raised.
Closes gh-4884

[ci skip]
Merge pull request #4905 from juliantaylor/doc-update

documentation updates