Documentation of self-alignment-X

[Richard Shann]

the docs (
http://www.lilypond.org/doc/v2.18/Documentation/internals/self_002dalignment_002dinterface
 ) say:
"
 self-alignment-X (number)

    Specify alignment of an object. The value -1 means left aligned, 0
centered, and 1 right-aligned in X direction. Other numerical values may
also be specified. 
"
I think it would be well worth adding to the last sentence:

The unit is half the object width.

Richard Shann