Update References Section in Style Guide

[lglattly]

This is to update a guideline about the References sections of docstrings in the new style guide.

CC @sympy/gsod

References to other Issues or PRs

Brief description of what is fixed or changed

Other comments

Release Notes

NO ENTRY

[sympy-bot]

✅

Hi, I am the SymPy bot (v149). I'm here to help you write a release notes entry. Please read the guide on how to write release notes.

• No release notes entry will be added for this pull request.

Note: This comment will be updated with the latest check if you edit the pull request. You need to reload the page to see it.

Click here to see the pull request description that was parsed.

This is to update a guideline about the References sections of docstrings in the new style guide.

CC @sympy/gsod

#### References to other Issues or PRs



#### Brief description of what is fixed or changed


#### Other comments


#### Release Notes



<!-- BEGIN RELEASE NOTES -->

NO ENTRY

<!-- END RELEASE NOTES -->

[asmeurer]

Looks good to me. I removed the release notes entry because this is a change to something that is new since the last release, so it doesn't need a separate entry.

[codecov]

Codecov Report

Merging #17863 into master will increase coverage by 8.325%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##            master    #17863       +/-   ##
=============================================
+ Coverage   66.498%   74.823%   +8.325%     
=============================================
  Files          638       638               
  Lines       166472    166472               
  Branches     39129     39129               
=============================================
+ Hits        110701    124560    +13859     
+ Misses       50223     36397    -13826     
+ Partials      5548      5515       -33