Add docstrings to functions in nlp.jl

[odow]

Part of #1825

[codecov]

Codecov Report

Merging #2468 (9467368) into master (c432f66) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2468   +/-   ##
=======================================
  Coverage   92.23%   92.23%           
=======================================
  Files          43       43           
  Lines        4652     4652           
=======================================
  Hits         4291     4291           
  Misses        361      361           

Impacted Files	Coverage Δ
src/JuMP.jl	78.49% <ø> (ø)
src/nlp.jl	92.33% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c432f66...9467368. Read the comment docs.