Update IpcDataCache documentation

Update the comment in front of IpcDataCache to mention the two most
frequently raised issues.  At the same time, replace the documentation
in PropertyInvalidatedCache with a pointer to IpcDataCache.

Test: doc-only
Flag: EXEMPT comments only
Bug: 363321822
Change-Id: I64f27da7f943bab957fca8960011014dd30cd7fe