Add warning and improve docs for modify-then-query

[odow]

Closes #2797

[codecov]

Codecov Report

Merging #2858 (9c264f0) into master (b7a84e5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 9c264f0 differs from pull request most recent head 65780e9. Consider uploading reports for the commit 65780e9 to get more accurate results

@@           Coverage Diff           @@
##           master    #2858   +/-   ##
=======================================
  Coverage   94.79%   94.79%           
=======================================
  Files          44       44           
  Lines        5709     5710    +1     
=======================================
+ Hits         5412     5413    +1     
  Misses        297      297           

Impacted Files	Coverage Δ
src/JuMP.jl	86.78% <100.00%> (+0.04%)	⬆️

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[odow]

Objections to merging?