Add 'Inverse scale' in gamma error output.

[doikov]

Submission Checklist

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

Summary:

gamma_* functions use shape and rate (inverse scale) parametrization for Gamma distribution.
Now the doc and the parameters checkers are consistent with the input parameters.

The issue: #514

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Nikita Doykov

By submitting this pull request, the copyright holder is agreeing to 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/)

[bob-carpenter]

@Doykov: Thanks for the fix.

We need a copyright declaration and an agreement by the copyright holder to release the code open source before we can merge.

[bob-carpenter]

The code looks great. We just need copyright/licensing sign off (just edit the original pull request) and this is good to go.

[doikov]

I've added the copyright.

make cpplint returns:
make: *** [cpplint] Error 1
as before the commit.
I didn't get why, but this pull request is only about comments so I think It's ready to be merged.