Bugfix/doxygen build doc fixes

[peterwicksstringfield]

Summary

Fixes #1543

Couple quick documentation fixups. The kernel files define two functions, and the documentation of the second is supposed to link to the documentation of the first. Looks like a copy-paste error using the code in the (no-longer-existent) subtract.hpp as a template.

Tests

None.

Side Effects

None.

Checklist

• Math issue a couple doxygen errors on develop #1543

• Copyright holder: Peter Wicks Stringfield

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[mcol]

Thank you! I don't get the error on the Contributing page, but for the README we could just add that to the EXCLUDE_PATTERNS line in doxygen/doxygen.cfg.

[stan-buildbot]

(stat_comp_benchmarks/benchmarks/gp_pois_regr/gp_pois_regr.stan, 0.97)
(stat_comp_benchmarks/benchmarks/low_dim_corr_gauss/low_dim_corr_gauss.stan, 1.02)
(stat_comp_benchmarks/benchmarks/irt_2pl/irt_2pl.stan, 1.0)
(stat_comp_benchmarks/benchmarks/pkpd/one_comp_mm_elim_abs.stan, 1.08)
(stat_comp_benchmarks/benchmarks/eight_schools/eight_schools.stan, 0.97)
(stat_comp_benchmarks/benchmarks/gp_regr/gp_regr.stan, 0.97)
(stat_comp_benchmarks/benchmarks/arK/arK.stan, 1.0)
(performance.compilation, 1.01)
(stat_comp_benchmarks/benchmarks/low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan, 1.01)
(stat_comp_benchmarks/benchmarks/low_dim_gauss_mix/low_dim_gauss_mix.stan, 1.0)
(stat_comp_benchmarks/benchmarks/sir/sir.stan, 0.99)
(stat_comp_benchmarks/benchmarks/pkpd/sim_one_comp_mm_elim_abs.stan, 1.01)
(stat_comp_benchmarks/benchmarks/garch/garch.stan, 1.0)
(stat_comp_benchmarks/benchmarks/gp_regr/gen_gp_data.stan, 1.0)
(stat_comp_benchmarks/benchmarks/arma/arma.stan, 0.97)
Result: 0.99888029125
Commit hash: d85fb6a

[rok-cesnovar]

Thanks!