This is not a feature but a reaction to the problem that with the new
and fixed OpenSmbold font used in OOo 3.3 it will be generally
impossible to always get the very same behavior for Greek characters
in Math as it was in OOo 3.2.
Thus it was decided to introduce a second set for Greek characters.
(Note: The word 'Greek' gets localized). One now consists of straight
(non-italic) charcters and the second is programmatically generated
from the first by adding italic versions of the characters in the
first to the second.
For more details see i113276.
This was regarded to be a usability fix for the new OOo 3.3, thus
there is also no spec for this one.