Adding clarification on how mixed_case_recommended_field works

[tzujenchanmbd]

Describe the problem

The current notice description of mixed_case_recommended_field warning is as follows:

This field contains customer-facing text and should use Mixed Case (upper and lower case letters)

However, it does not completely describe how validator works and cause potential confusions.

For example, a stop name 23rd and FM 107 will trigger the warning, since the validator would split text string based on non-letter characters into separate text string, but it does contain upper and lower case if looking at it as a whole.

Suggest a fix or an alternative

Adding clarification text on notice description.