Update docstrings to state usage of border padding

[lxvm]

The filter routines stated that they used zero padding of the design variables in convolution filters where they actually used border padding. I updated docstrings to reflect this.

[stevengj]

It would be good to expand the docs to precisely define "border padding".

[lxvm]

Thanks, I updated the docstrings to explain the implementation and purpose of border padding

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.89%. Comparing base (f29a8c7) to head (da9410e).
⚠️ Report is 96 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3123      +/-   ##
==========================================
+ Coverage   73.81%   73.89%   +0.08%     
==========================================
  Files          18       18              
  Lines        5423     5452      +29     
==========================================
+ Hits         4003     4029      +26     
- Misses       1420     1423       +3     

Files with missing lines	Coverage Δ
python/adjoint/filters.py	70.37% <ø> (+0.60%)	⬆️

... and 2 files with indirect coverage changes

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