Updated SegmentPackageManager docstrings. MILESTONE.

MILESTONE: There are 48 SegmentPackageManager commands. The are 48
SegmentPackageManager public methods. There are 48 SegmentPackageManager
test files. There is now an exact 1:1:1 correspondence between UI commands,
public methods and test files for the SegmentPackageManager. This is the first
class in the system to reach this point.

The 48 SegmentPackageManager commands are these:

    Red Example Score (2013) - segments - A - available commands

        __init__.py - open (ipyo)             repository - add (rad)
        __init__.py - write stub (ipyws)      repository - clean (rcn)
                                              repository - commit (rci)
        __make__.py - interpret (mpyi)        repository - revert (rrv)
        __make__.py - open (mpyo)             repository - status (rst)
        __make__.py - write stub (mpyws)      repository - update (rup)

        __metadata__.py - open (mdpyo)        system - LilyPond log (log)
        __metadata__.py - rewrite (mdpyrw)    system - Python (pyi)
                                              system - available commands (?)
        definition.py - edit (dpye)           system - doctest (pyd)
        definition.py - write stub (dpyws)    system - pytest (pyt)
                                              system - session variables (sv)
        edit - score stylesheet (ess)         system - shell (!)

        go - back (b)                         versions - definition.py - open (vdpyo)
        go - home (h)                         versions - output.ly - open (volyo)
        go - quit (q)                         versions - output.pdf - open (vopdfo)
        go - score (s)

        go - next score (>>)
        go - next asset (>)
        go - previous score (<<)
        go - previous asset (<)

        go - build (u)
        go - distribution (d)
        go - makers (k)
        go - materials (m)
        go - segments (g)
        go - stylesheets (y)

        metadata - add (mda)
        metadata - get (mdg)
        metadata - remove (mdrm)

        output.ly - interpret (olyi)
        output.ly - open (olyo)

        output.pdf - open (opdfo)

        package - version (ver)
        package - versions list (verls)

    >

The other classes will follow. The order will be managers first and wranglers
after.