[docs] fix and test docstrings

[odow]

Closes #3442

Still a WIP, because there a few things I need to fix.

[codecov]

Codecov Report

Patch coverage: 66.66% and project coverage change: -0.02% ⚠️

Comparison is base (a325eb6) 98.01% compared to head (4cae778) 97.99%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3445      +/-   ##
==========================================
- Coverage   98.01%   97.99%   -0.02%     
==========================================
  Files          36       36              
  Lines        5039     5041       +2     
==========================================
+ Hits         4939     4940       +1     
- Misses        100      101       +1     

Files Changed	Coverage Δ
src/Containers/DenseAxisArray.jl	96.48% <ø> (ø)
src/Containers/SparseAxisArray.jl	98.71% <0.00%> (-0.64%)	⬇️
src/Containers/container.jl	94.33% <ø> (ø)
src/Containers/macro.jl	100.00% <ø> (ø)
src/JuMP.jl	94.87% <ø> (ø)
src/constraints.jl	96.57% <ø> (ø)
src/copy.jl	96.19% <ø> (ø)
src/feasibility_checker.jl	100.00% <ø> (ø)
src/lp_sensitivity2.jl	97.79% <ø> (ø)
src/macros.jl	98.67% <ø> (ø)
... and 6 more

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