[DOC] document additional generic driver configuration options

Dear developers,

thanks for your hard work with openstack manila. It is really appreciated.

This bug report refers to the documentation of the project. We've been reading the documentation, and detected this:

* some drivers have extensive documentation on how to configure/setup them, for example ZFS on Linux: https://docs.openstack.org/manila/latest/admin/zfs_on_linux_driver.html#backend-configuration This includes the configuration options that make sense for manila.conf and such.

* some other drivers, for example the generic one, lacks this information: https://docs.openstack.org/manila/latest/admin/generic_driver.html

Several configuration modes are mentioned in that document, but they aren't described anywhere.

Specifically, it would be great if you can include documentation on how configure the share backend when using:

=== 8< ===
[mybackend1]
share_driver = manila.share.drivers.generic.GenericShareDriver
driver_handles_share_servers = False
# what other options are expected here?

[mybackend2]
share_driver = manila.share.drivers.generic.GenericShareDriver
driver_handles_share_servers = True
# what other options are expected here?
=== 8< ===

In the example manila.conf file, each configuration option has a small comment like this:

=== 8< ===
# Name or ID of service instance in Nova to use for share exports.
# Used only when share servers handling is disabled. (string value)
#service_instance_name_or_id = <None>
=== 8< ===

But is not enough to get a general overview of what we are supposed to configure. For someone new to openstack manila this guessing game can be discouraging.

Would it be possible to get a sample manila.conf file for both cases? (I mean DHSS=false and DHSS=true)