DOC: Update documentation for InTemporaryDirectory to mark it as not thread-safe

[jacobdr]

Closes #1102

[codecov]

Codecov Report

Merging #1103 (420164a) into master (7cfaebf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1103   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files         100      100           
  Lines       12247    12247           
  Branches     2393     2393           
=======================================
  Hits        11303    11303           
  Misses        621      621           
  Partials      323      323           

Impacted Files	Coverage Δ
nibabel/tmpdirs.py	92.10% <ø> (ø)

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[matthew-brett]

Thanks - looks fine.