Update documentation for Storage Profiles (#299)

* Update documentation for Storage Profiles

The storage profiles documentation was stale, and many fields weren't documented. This
commit adds documentation for existing fields as well as the new UserCommands and AllocationPadding
fields.

Signed-off-by: Matt Richerson <matthew.richerson@hpe.com>

* add documentation for which variables are available during user command execution

Signed-off-by: Matt Richerson <matthew.richerson@hpe.com>

* Add documentation about user command output

Signed-off-by: Matt Richerson <matthew.richerson@hpe.com>

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Signed-off-by: Matt Richerson <matthew.richerson@hpe.com>

Author: matthew-richerson