Clarify and expand management interface documentation

Clarify and expand management interface documentation

Clarify and expand the documentation for the management interface:

* Add examples of static and dynamic challenge/response sequences in
the "COMMAND -- password and username" section.

* Expand the "Challenge/Response" section with more detail.

* Use "management interface client" throughout (instead of "management
client", which was used in several places previously).

* Clarify when both a username and password are needed, not just a
username or a password.

* Clarify that an exit with a fatal error for a dynamic C/R will occur
only if "--auth-retry none" (the default) is in effect.

* Fix a typo. ("posesses" => "possesses").

Signed-off-by: Jonathan K. Bullard <jkbullard@gmail.com>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <nEV9l80I3peitTd26qmQFpeoaQbEO-IR74B1gOvCLv-IfvQKjNfL9UnZq1aWr20480nGcbkSnhA-mSGEI5kG7JBMsGpNbNf2FExV3CSzRf4=@protonmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17390.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit a6fd48ba36ede465b0905a95568c3ec0d425ca71)