DOC: Standardize module-level docstrings format

[ZviBaratz]

The majority of nibabel's docstrings do not contain a period in the summary. The exceptions stick out a little in the API docs. This PR fixes these exceptions and homogenizes spacing within and after module-level docstrings.

[codecov]

Codecov Report

Base: 92.14% // Head: 92.16% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (7903364) compared to base (3f1c6f9).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1200      +/-   ##
==========================================
+ Coverage   92.14%   92.16%   +0.02%     
==========================================
  Files          97       97              
  Lines       12332    12332              
  Branches     2534     2534              
==========================================
+ Hits        11363    11366       +3     
+ Misses        647      645       -2     
+ Partials      322      321       -1     

Impacted Files	Coverage Δ
nibabel/arraywriters.py	96.86% <ø> (ø)
nibabel/deprecated.py	100.00% <ø> (ø)
nibabel/deprecator.py	100.00% <ø> (ø)
nibabel/ecat.py	88.21% <ø> (ø)
nibabel/eulerangles.py	95.00% <ø> (ø)
nibabel/fileholders.py	93.33% <ø> (ø)
nibabel/filename_parser.py	92.04% <ø> (ø)
nibabel/fileslice.py	97.26% <ø> (ø)
nibabel/imageclasses.py	100.00% <ø> (ø)
nibabel/imageglobals.py	90.90% <ø> (ø)
... and 25 more

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[effigies]

Thanks!