BUG: io.netcdf: do not close mmap if there are references left to it

Fixes gh-3630
DOC: io: update netcdf docs vs mmap
DOC: io/netcdf: add example on mmap features, adjust doc

[ci skip]
TST: io/netcdf: remove invalid test

The "Too many open files" warning is actually unavoidable, since the
arrays refer directly to data in memory mapped files, and those files
cannot be closed as long as the arrays are around.
TST: io/netcdf: fix issues with warnings in tests

Ignore warning emitted in test_itemset_no_segfault_on_readonly.  Remove
a test using assert_warns; it doesn't clear warning registry, so the
warning in the other test makes this one not fire.
Merge pull request #3960 from pv/netcdf-fix

BUG: io.netcdf: do not close mmap if there are references left to it