logging: add examples for logging.

As suggested by @cdecker, but also did a bit of minor reformatting.

I don't like the excessive indenting in our man pages though,
but that's a separate problem.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/lightningd-config.5

@@ -159,9 +159,14 @@ internal integer id assigned for the lifetime of the channel:
 .IP \[bu]
 \fIonchaind-chan #N\fR: Each onchain close handling daemon
 .IP \[bu]
-
 \fIjson #FD\fR: Each JSONRPC connection, FD = file descriptor number
 
+.RE
+
+  So, \fBlog-level=debug:plugin\fR would set debug level logging on all
+plugins and the plugin manager\.  \fBlog-level=io:chan #55\fR would set
+IO logging on channel number 55 (or 550, for that matter)\.
+
 
  \fBlog-prefix\fR=\fIPREFIX\fR
 Prefix for log lines: this can be customized if you want to merge logs
@@ -200,9 +205,6 @@ If set, you will be prompted to enter a password used to encrypt the \fBhsm_secr
 Note that once you encrypt the \fBhsm_secret\fR this option will be mandatory for
 \fBlightningd\fR to start\.
 
-
-
-.RE
 .SH Lightning node customization options
 
  \fBalias\fR=\fIRRGGBB\fR

doc/lightningd-config.5.md

@@ -118,6 +118,7 @@ for any subsystem containing that string.  Subsystems include:
 * *gossipd*: The gossip daemon
 * *connectd*: The network connection daemon
 
+
   The following subsystems exist for each channel, where N is an incrementing
 internal integer id assigned for the lifetime of the channel:
 * *openingd-chan #N*: Each opening / idling daemon
@@ -126,6 +127,11 @@ internal integer id assigned for the lifetime of the channel:
 * *onchaind-chan #N*: Each onchain close handling daemon
 * *json #FD*: Each JSONRPC connection, FD = file descriptor number
 
+
+  So, **log-level=debug:plugin** would set debug level logging on all
+plugins and the plugin manager.  **log-level=io:chan #55** would set
+IO logging on channel number 55 (or 550, for that matter).
+
  **log-prefix**=*PREFIX*
 Prefix for log lines: this can be customized if you want to merge logs
 with multiple daemons.