doc: update decode instruction for emergency.recover

Related to:
ElementsProject#6711
ElementsProject#6773

Changelog-None

doc/lightning-decode.7.md

@@ -15,7 +15,9 @@ The **decode** RPC command checks and parses:
   or `LIGHTNING:`) as specified by the BOLT 11 and BOLT 12
   specifications.
 - a *rune* as created by lightning-commando-rune(7).
-- a *emergency\_recover* starting with clnemerg1 which contains the content of emergency.recover file.
+- an *emergency\_recover* string generated by hsmtool like
+ `lightning-hsmtool getemergencyrecover <path/to/emergency.recover>`.
+  It holds `emergency.recover` contents and starts with `clnemerg1`.
 
 It may decode other formats in future.