Allow creating Job Alerts based on the WPJM shortcode restrictions

### Is your feature request related to a problem? Please describe
When you restrict the visibilities of job listings using shortcodes, job alerts created from such jobs listings still pull from entire database.

For example, say a user created two separate job listings pages.

- A page with `[jobs]` shortcode show both featured and regular jobs.
- Another page with `[jobs featured="false"]` only show regular jobs, excluding featured jobs.

From these job listing pages, you can filter the result by job type. Then `Add alert` button shows up. The alerts created this way includes all jobs instead of featured, or not featured jobs as the shortcode specified.


### Describe the solution you'd like

@gaurav1984 suggested the following:

>We can adopt some of the same arguments used in the [jobs] shortcode for the [job_alerts] shortcode.
This way the user can setup different alert pages ( access managed by their membership plugin ) where candidates can create new alerts for themselves choosing jobs only from the subset specified by the argument in the short-code.

### Describe alternatives you've considered

Updating the documentation to clarify the scope of the shortcode in relation to job alerts

### Additional context
4456832-zd-woothemes

Start with the [jobs] and [job_alerts] shortcode implementations and trace how visibility arguments become the job-alert query. Compare the supported [jobs] restrictions with the alert flow; done means alerts created from a restricted listing page include only the specified subset, with documentation updated if that is the chosen alternative.

Written by the indexing model from the issue text.