Doc clarifications

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Closes #1051
Closes #1052

• Fixes a mistake in the the loo method documentation (variables can actually only contain a single variable).
• Adds some clarifying comments in the optimize and CmdStanMLE doc about the jacobian adjustment.

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):
Columbia University

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/)

[jgabry]

These are really minor doc tweaks, so going to go ahead and merge. The failure on macOS-latest (devel) is unrelated.