Small fixes to docs, json_dev_forget_channel, json_fundchannel and trailing space in logline

[ghost]

Hope this is allowed to bundle in single PR, can make separate if needed.

• doc: improve explanation of getroute command
  (as commented in PR Various fixes to fix the "we don't choose direct routes" problem. #2307)
• doc: fundchannel command, clarify optional feerate
  (mentioned in issue Channel closing because local/remote feerates are too different #2354)
• remove trailing space in logline and small coding style fix
  (as commented in PR json_withdraw: fix change_outputs showing forever as unconfirmed in listfunds #2310)
• lightningd/json_dev_forget_channel: clarify message when both peer_id and scid are given
  (mentioned in issue Opening a channel with "tady je slushovo" (still) crashes my node #2298)
• lightningd/json_fundchannel: use json_add_string to add hex_tx to response, similar as in json_withdraw

[cdecker]

ACK e3ea65a