Clarifying comments in model_base, namely 'set'-->'appended' parameters

[wrongu]

Submission Checklist

• Run unit tests: ./runTests.py src/test/unit
• Run cpplint: make cpplint
• Declare copyright holder and open-source license: see below

Summary

Minor documentation edit. I ran into issues extending the stan::model::model_base class due to misinterpreting the role of get_param_names, constrained_param_names, and unconstrained_param_names.

[stan-buildbot]

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Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	3.56	3.55	1.0	0.24% faster
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.96	-4.05% slower
eight_schools/eight_schools.stan	0.08	0.09	0.98	-1.64% slower
gp_regr/gp_regr.stan	0.14	0.14	1.0	-0.02% slower
irt_2pl/irt_2pl.stan	5.69	5.7	1.0	-0.16% slower
performance.compilation	92.89	90.72	1.02	2.34% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.14	8.1	1.01	0.5% faster
pkpd/one_comp_mm_elim_abs.stan	32.18	31.03	1.04	3.58% faster
sir/sir.stan	121.32	120.7	1.01	0.51% faster
gp_regr/gen_gp_data.stan	0.03	0.03	1.0	-0.12% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	3.0	3.03	0.99	-0.99% slower
pkpd/sim_one_comp_mm_elim_abs.stan	0.39	0.37	1.03	2.87% faster
arK/arK.stan	2.04	2.05	1.0	-0.4% slower
arma/arma.stan	0.27	0.28	0.99	-0.83% slower
garch/garch.stan	0.61	0.62	0.99	-1.26% slower
Mean result: 1.00070265734

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Jenkins Console Log
Blue Ocean
Commit hash: 93e8430

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Machine information

ProductName: Mac OS X ProductVersion: 10.11.6 BuildVersion: 15G22010

CPU:
Intel(R) Xeon(R) CPU E5-1680 v2 @ 3.00GHz

G++:
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix

Clang:
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix