doc: Update developers-guide

- communicate regarding YANG
- add StatD with focus on Yanger
- add test dev procedures

Resolves #934

doc/developers-guide.md

@@ -166,22 +166,33 @@ Now you can rebuild `confd`, just as described above, and restart Infix:
 
 ### `statd`
 
-The Infix `src/statd/` (Status Daemon) populates operational data in 
-the sysrepo datastore using again XPath subscriptions. 
-
-StatD relies entirely on `yanger`, a Python script that gathers data from 
-local linux services and feeds it into sysrepo.
+The Infix status daemon, `src/statd`, is responsible for populating the 
+sysrepo `operational` datastore. Like `confd` it uses XPath subscriptions, 
+but unlike confd it relies entirely on `yanger`, a Python script that 
+gathers data from local linux services and feeds it into sysrepo.
 
 To apply changes, rebuild the image:
 
     make python-statd-rebuild statd-rebuild all
 
-Rebuilding statd for every change during development process can be 
-tedious. Instead, `yanger` allows remote execution, running the script
-directly on the host system (test container):
+Rebuilding the image and testing on target for every change during 
+development process can be tedious. Instead, `yanger` allows remote 
+execution, running the script directly on the host system (test 
+container):
 
     infamy0:test # ../src/statd/python/yanger/yanger -x "../utils/ixll -A ssh d3a" ieee802-dot1ab-lldp
 
+`ixll` is a utility script that enables network commands to be executed
+using an interface name instead of a hostname. It simplifies operations 
+like `ssh`, `scp`, and network discovery. 
+
+`yanger` normally runs commands on the local systems to retrieve data 
+(i.e. `lldpcli`). When executed with `-x "../utils/ixll -A ssh d3a"`,
+it redirects those commands to run on a remote system 
+(connected to local `d3a` interaface) via SSH. This is aimed for remote
+execution of `yanger` in 
+[interractive test environment](testing.md#interactive-usage)
+
 For debugging or testing, you can capture system command output and 
 replay it later without needing a live system.
 

doc/testing.md

@@ -88,7 +88,7 @@ arguments:
 To run a suite of tests, e.g., only the DHCP client tests, pass the
 suite as an argument to [9PM][]:
 
-    11:42:53 infamy0:test # ./9pm/9pm.py case/infix_dhcp/all.yaml
+    11:42:53 infamy0:test # ./9pm/9pm.py case/infix_dhcp/infix_dhcp.yaml
 
 To run the suite of all tests: