Add documentation for std_normal_{cdf,lcdf,lccdf,rng}

[mcol]

Submission Checklist

• Builds locally
• Declare copyright holder and open-source license: see below

Summary

This documents the functions that are being added as part of stan-dev/math#1580. This should be committed only after that has been fixed and the new functions have been exposed to the language.

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):

Marco Colombo

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]

Looks good. And thanks for adding the functions that required this doc update!