[docs] Add missing docstrings

[odow]

Part of #1825

[codecov]

Codecov Report

Merging #2414 (44247d3) into master (52b1b38) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2414      +/-   ##
==========================================
- Coverage   92.19%   92.18%   -0.01%     
==========================================
  Files          43       43              
  Lines        4597     4595       -2     
==========================================
- Hits         4238     4236       -2     
  Misses        359      359              

Impacted Files	Coverage Δ
src/aff_expr.jl	95.03% <ø> (ø)
src/macros.jl	92.13% <ø> (ø)
src/objective.jl	91.66% <ø> (ø)
src/optimizer_interface.jl	71.42% <ø> (ø)
src/quad_expr.jl	97.43% <ø> (ø)
src/variables.jl	98.46% <ø> (ø)
src/constraints.jl	93.42% <100.00%> (-0.09%)	⬇️

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