mutation_reader_test: add description to multishard-tests
mutation_reader_test: add description to multishard-tests

These tests are quite complicated and require intimate knowledge of how
foreign_reader and multishard_combining_reader operates. Knowing these
two objects is still required to understand the tests but make it that
much easier by explaining how they were designed to test what they test.

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <8de580131a8652924de920c2bc68a98e579398ee.1525328226.git.a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,duarten/scylla