Improve documentation about packet decode characters

[dranch]

Hello John,

In the User Guide, please consider either duplicating, moving or cross-referencing the section detailing the Direwolf modem decoding repair characters in Section 9.2.4 starting with the line "When multiple modems are configured per channel" concluding with "which modem was used" to Section 12.3 which covers the FIX_BITS feature. It was difficult to find this information when a user was trying to understand what Direwolf's STDOUT was telling him about the modem's decoder success/failure.

--David