CRM-20180 - Documentation URLs should contain "stable" *or* "current", not both

CRM-20180 - Documentation URLs should contain "stable" *or* "current", not both

Examples:

 * In "Administer => Communications => Scheduled Reminders", the green box has
   a link to "learn more". This links to "https://docs.civicrm.org/user/en/stable/current/email/scheduled-reminders/"
 * In "Search =>= Advanced Search", view the help for "Views for Display Contacts".
   The help message includes a link to "https://docs.civicrm.org/user/en/stable/current/organising-your-data/profiles"

Both examples include the redundant formulation "user/en/stable/current" and
point to non-existent pages.  Use either "stable" or "current".  It would be
silly for one installation to contain a mix of links for "stable" and
"current".  It appears that `formatDocUrl()` aims to normalize these.