fixed some miss communication about _replicator

Was saying to `PUT` to `_replicator` directly.
Which throws a "database exists" or similar error.

Also tweaked intro copy to be clearer.

Signed-off-by: Alexander Shorin <kxepal@apache.org>
Docs: add synopsis, deprecated and noindex options to HTTP endpoints

Synopsis is the short description of the HTTP endpoint that used in
the generated API reference.

Deprecated option, if specified, adds special remark to the endpoint
description in the same reference.

Noindex excludes endpoint from the reference.

The old source of synopsises is removed.