Docs for SERVER-22663: Make --shardsvr required for a mongod to be used as a shard

Engineering Ticket Description:

3.4 onwards:

a shard can become shard aware in two ways:

• started with --shardsvr and shardIdentity document found on disk
• started with --shardsvr and receive shardIdentity insert

so, we will eventually reject all sharding commands if:

• not started with --shardsvr
• started with --shardsvr but not yet sharding aware

=============

for backwards compatibility with 3.2:

a shard can also become shard aware if:

• started with --shardsvr and receive command that initializes sharding awareness

so (what this ticket does):

reject all sharding commands except a whitelist of commands that initialize sharding awareness.