PG-1327: Added check for allowSplitting flag before accessing the m_modifiedBooks collection and made explicit in the stated contract of the MatchVernBlocksToReferenceTextBlocks method the conditions under which the caller is responsible for locking m_modfiedBooks.
PG-1327: Added check for allowSplitting flag before accessing the m_modifiedBooks collection and made explicit in the stated contract of the MatchVernBlocksToReferenceTextBlocks method the conditions under which the caller is responsible for locking m_modfiedBooks.

Added comments to clarify the purpose/behavior of some ReferenceText methods

sillsdev/Glyssen,sillsdev/Glyssen