clarify level 2 rules for Greek ligatures and abbreviation glyphs

[bertsky]

The current formulation is quite vague on Greek, but the corresponding precise ruleset for Latin (vocalic yes, consonantal no) suggests that we should indeed try to "regularize" or "canonicalize" the ligatures and abbreviation glyphs used pervasively in hellenistic times (at least those surviving until the 19th century and thus receiving a Unicode codepoint).

In particular,

• ȣ → ου
• ϛ → στ (i.e. not ς!)
• Ϛ → ΣΤ
• ...
• ϗ → καί
• Ϗ → Καί
• ...

or is this only relevant on level 1?