voicemail.conf: Fix incorrect comment about #include.

A comment at the top of voicemail.conf says that #include
cannot be used in voicemail.conf because this breaks
the ability for app_voicemail to auto-update passwords.
This is factually incorrect, since Asterisk has no problem
updating files that are #include'd in the main configuration
file, and this does work in voicemail.conf as well.

ASTERISK-30479 #close

Change-Id: I3bf7d275849ab83f55f7fb6702a75a3077ee1df3
(cherry picked from commit d0f39250eeb7e62496477fd4cf57827eb9a99624)