Docs: Update login chapter

Docs: Update login chapter
Some things have changed in the web client since this documentation was
first created. Removing the note that workstation registration is not required
and updating the caution on what happens when exiting the browser.

Previously, workstation registration was part of installing the staff client,
but since we have nothing to install, I added it to the logging in section. I
also removed a xul client feature from the main root.adoc.

Signed-off-by: Kathy Lussier <klussier@masslnc.org>
Conflicts:
	docs/admin/web_client-login.adoc