Clarify doc for `variable` argument to `extract_variable()` and `extract_variable_matrix()`

[jgabry]

Summary

Very minor doc edit. Adds an additional emphasis to the doc that the variable argument must correspond to a scalar. Some confusion about this was brought up in the Stan meeting today.

Copyright and Licensing

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]

Failures are unrelated to the PR