memtx: make tuple compare hints optional

Since 9fba29ab (memtx: introduce tuple
compare hint) memtx tree key data (indexes) started to contain extra 8
bytes as a hint. Now it is optional and can be turned off in an index
options with "hint = false" entry.

Closes #4927

@TarantoolBot document
Title: memtx: optional tuple compare hints
Update the documentation for an index creation to reflect that there is
now an option to turn off hints for the index.