Feature/issue 891 check cov matrix nan

#### Summary:
Added NaN test and doc in code for check_cov_matrix. This should not be merged before #905 or #901.

#### Intended Effect:
Added NaN test and doc in code for check_cov_matrix. 

#### How to Verify:
Run test/unit/math/error_handling/matrix/check_cov_matrix

#### Side Effects:
None.

#### Documentation:
Added doc in the code.

#### Reviewer Suggestions:
Anyone.