Document session logout, "include admin" and notify behavior

Problem/Motivation

The README, help page and form don't explain some behavior that
surprises admins:

• Users whose passwords are reset are logged out of all sessions
  (core deletes a user's sessions when their password changes). This
  includes uid 1 when "Include admin user" is checked.
• "Include admin user" has no effect when uid 1 runs the reset, because
  the user running the reset is never included. To reset uid 1, run it
  from another admin account.
• Emails are only sent when "Notify active users" or "Notify blocked
  users" is checked.

Also in the same text:

• Typo "authenicated" in README.txt and the help page (flagged by
  cspell).
• The help page link to the form is hard-coded to
  /admin/people/mass-pwreset, which breaks on sites installed in a
  subdirectory.
• The "Password Reset Options" list on the help page is not
  translatable.

Proposed resolution

• Update README.txt, the help page and the form descriptions to
  cover the three behaviors above.
• Fix the typo.
• Build the help page link with Url::fromRoute().
• Wrap the help list items in t().

Remaining tasks

• Create MR
• Review

User interface changes

New/updated descriptions on the Mass Password Reset form and help
page.

API changes

None.

Data model changes

None.