Add more docstrings

[odow]

Part of #1825

[codecov]

Codecov Report

Merging #2547 (24f51db) into master (3e517d7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2547   +/-   ##
=======================================
  Coverage   92.94%   92.94%           
=======================================
  Files          44       44           
  Lines        5456     5456           
=======================================
  Hits         5071     5071           
  Misses        385      385           

Impacted Files	Coverage Δ
src/JuMP.jl	82.30% <ø> (ø)
src/aff_expr.jl	95.83% <ø> (ø)
src/constraints.jl	94.97% <ø> (ø)
src/macros.jl	92.91% <ø> (ø)
src/print.jl	93.34% <ø> (ø)
src/quad_expr.jl	97.81% <ø> (ø)
src/sets.jl	90.47% <ø> (ø)
src/variables.jl	98.77% <ø> (ø)

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