Remove obsolete messages
Update po
Add missing po files
Translate error information
Describe type and body of command messages in a unified format
Change heading of command references to more meaningful

Syntax => Parameter syntax
Parameters => Parameter details
Describe sample requests and responses as Droonga messages
Add notes about response statuses of Groonga compatible commands
Show sample requests and responses as Droonga messages
Add error information for "add" command
Indicate error status codes
Add more error infromation about "add" command
Add error information for "search" command
Fix typo

an table =>
a table
Add more error definitions for "search" command and around the message itself.
Add architecture overview image

TODO: Draw Engine internal.
Link to general errors from error information of each command
Fix typo: too much "s"
Fix typo
Fix a typo

follwing ->
following