[Book] Add more information about some of the pmclass flags "abstract", "has_ro", "is_ro", and "is_shared". Many of these are almost completely undocumented elsewhere, so my descriptions here might not be completely accurate.
[Book] Add more information about some of the pmclass flags "abstract", "has_ro", "is_ro", and "is_shared". Many of these are almost completely undocumented elsewhere, so my descriptions here might not be completely accurate.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@33918 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot