doc: properly document sub-subobject fields.

Inside listpeerchannels, there's an object called `updates`, and inside that objects `local` and `remote`.  We flatten this, but the documentation doesn't mention the `updates` field at all.

Ideally we would rename these fields to include `updates_` but that would be a breaking change.

Signed-off-by: Rusty Russell <[email protected]>

```
     - `row` (reference to `peerchannels_channel_type.rowid`, sqltype `INTEGER`)
     - `arrindex` (index within array, sqltype `INTEGER`)
     - `names` (type `string`, sqltype `TEXT`)
-  - `local_htlc_minimum_msat` (type `msat`, sqltype `INTEGER`, from JSON object `local`)
-  - `local_htlc_maximum_msat` (type `msat`, sqltype `INTEGER`, from JSON object `local`)
-  - `local_cltv_expiry_delta` (type `u32`, sqltype `INTEGER`, from JSON object `local`)
-  - `local_fee_base_msat` (type `msat`, sqltype `INTEGER`, from JSON object `local`)
-  - `local_fee_proportional_millionths` (type `u32`, sqltype `INTEGER`, from JSON object `local`)
-  - `remote_htlc_minimum_msat` (type `msat`, sqltype `INTEGER`, from JSON object `remote`)
-  - `remote_htlc_maximum_msat` (type `msat`, sqltype `INTEGER`, from JSON object `remote`)
-  - `remote_cltv_expiry_delta` (type `u32`, sqltype `INTEGER`, from JSON object `remote`)
-  - `remote_fee_base_msat` (type `msat`, sqltype `INTEGER`, from JSON object `remote`)
-  - `remote_fee_proportional_millionths` (type `u32`, sqltype `INTEGER`, from JSON object `remote`)
+  - `local_htlc_minimum_msat` (type `msat`, sqltype `INTEGER`, from JSON object `updates.local`)
+  - `local_htlc_maximum_msat` (type `msat`, sqltype `INTEGER`, from JSON object `updates.local`)
+  - `local_cltv_expiry_delta` (type `u32`, sqltype `INTEGER`, from JSON object `updates.local`)
+  - `local_fee_base_msat` (type `msat`, sqltype `INTEGER`, from JSON object `updates.local`)
+  - `local_fee_proportional_millionths` (type `u32`, sqltype `INTEGER`, from JSON object `updates.local`)
+  - `remote_htlc_minimum_msat` (type `msat`, sqltype `INTEGER`, from JSON object `updates.remote`)
+  - `remote_htlc_maximum_msat` (type `msat`, sqltype `INTEGER`, from JSON object `updates.remote`)
+  - `remote_cltv_expiry_delta` (type `u32`, sqltype `INTEGER`, from JSON object `updates.remote`)
+  - `remote_fee_base_msat` (type `msat`, sqltype `INTEGER`, from JSON object `updates.remote`)
+  - `remote_fee_proportional_millionths` (type `u32`, sqltype `INTEGER`, from JSON object `updates.remote`)
   - `ignore_fee_limits` (type `boolean`, sqltype `INTEGER`)
   - `lost_state` (type `boolean`, sqltype `INTEGER`)
   - `feerate_perkw` (type `u32`, sqltype `INTEGER`, from JSON object `feerate`)
```

plugins/sql.c

@@ -1581,6 +1581,14 @@ static const char *fmt_indexes(const tal_t *ctx, const char *table)
 	return tal_fmt(ctx, " indexed by `%s`", ret);
 }
 
+static const char *json_prefix(const tal_t *ctx,
+			       const struct table_desc *td)
+{
+	if (td->is_subobject)
+		return tal_fmt(ctx, "%s%s.", json_prefix(tmpctx, td->parent), td->cmdname);
+	return "";
+}
+
 static void print_columns(const struct table_desc *td, const char *indent,
 			  const char *objsrc)
 {
@@ -1603,7 +1611,8 @@ static void print_columns(const struct table_desc *td, const char *indent,
 				const char *subobjsrc;
 
 				subobjsrc = tal_fmt(tmpctx,
-						    ", from JSON object `%s`",
+						    ", from JSON object `%s%s`",
+						    json_prefix(tmpctx, td),
 						    td->columns[i]->jsonname);
 				print_columns(subtd, indent, subobjsrc);
 			}
@@ -1613,7 +1622,8 @@ static void print_columns(const struct table_desc *td, const char *indent,
 		if (streq(objsrc, "")
 		    && td->columns[i]->jsonname
 		    && !streq(td->columns[i]->dbname, td->columns[i]->jsonname)) {
-			origin = tal_fmt(tmpctx, ", from JSON field `%s`",
+			origin = tal_fmt(tmpctx, ", from JSON field `%s%s`",
+					 json_prefix(tmpctx, td),
 					 td->columns[i]->jsonname);
 		} else
 			origin = "";