Update docstring for fit measures

[alyst]

Expand a bit the docstrings for the fit measures.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.39%. Comparing base (1f8d2a9) to head (d12bf4d).
⚠️ Report is 30 commits behind head on devel.

Additional details and impacted files

@@            Coverage Diff             @@
##            devel     #310      +/-   ##
==========================================
- Coverage   71.83%   71.39%   -0.45%     
==========================================
  Files          51       51              
  Lines        2223     2283      +60     
==========================================
+ Hits         1597     1630      +33     
- Misses        626      653      +27     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Maximilian-Stefan-Ernst]

Suggested change

	The degrees of freedom for the SEM with *N* observed variables is the difference
	between the number of parameters required to define the multivariate normal
	distribution of observed variables (*½N(N+1)* for the *N×N* covariance matrix,
	plus *N* for the means vector if present), and the number of model parameters
	(`nparams(model)`](@ref nparams)).
	The degrees of freedom for the SEM with *N* observed variables is the difference
	between the number of observed data points (*½N(N+1)* for the *N×N* covariance matrix,
	plus *N* for the means vector if present), and the number of model parameters
	(`nparams(model)`](@ref nparams)).

[Maximilian-Stefan-Ernst]

That's very helpful, I added on emore comment, otherwise I'm happy to merge.