Corrected a typo (input_headers should read input_header in search_command.py, line 509) and added docs for SearchCommand.input_header and SearchCommand.messages.

Sphinx requires that instance attributes be documented on each of the SearchCommand classes:

	GeneratingCommand, ReportingCommand, and StreamingCommand.

Backlog: Turn input_header and messages into properties.
Signed-off-by: David Noble <dnoble@splunk.com>