fix(plotutil.py, grid.py): fix .plot() method using mflay parameter

[jlarsen-usgs]

• update docstrings in Grid() class

#1132
#1119

[codecov]

Codecov Report

Merging #1134 (2e60cdf) into develop (4cfb11d) will increase coverage by 2.054%.
The diff coverage is n/a.

❗ Current head 2e60cdf differs from pull request most recent head 555ac08. Consider uploading reports for the commit 555ac08 to get more accurate results

@@              Coverage Diff              @@
##           develop     #1134       +/-   ##
=============================================
+ Coverage   72.050%   74.105%   +2.054%     
=============================================
  Files          225       221        -4     
  Lines        51941     49933     -2008     
=============================================
- Hits         37424     37003      -421     
+ Misses       14517     12930     -1587     

Impacted Files	Coverage Δ
flopy/discretization/grid.py	73.056% <ø> (-0.519%)	⬇️
flopy/plot/plotutil.py	44.206% <ø> (+0.274%)	⬆️
flopy/mt3d/mtsft.py	6.569% <0.000%> (-72.263%)	⬇️
flopy/modflow/mfswi2.py	62.222% <0.000%> (-10.086%)	⬇️
flopy/plot/crosssection.py	52.186% <0.000%> (-8.040%)	⬇️
flopy/modflow/mfoc.py	91.468% <0.000%> (-3.217%)	⬇️
flopy/utils/formattedfile.py	88.461% <0.000%> (-2.565%)	⬇️
flopy/utils/mfreadnam.py	75.182% <0.000%> (-2.190%)	⬇️
flopy/modflow/mfbcf.py	88.324% <0.000%> (-1.625%)	⬇️
flopy/utils/datafile.py	72.811% <0.000%> (-1.383%)	⬇️
... and 115 more