docs/getting-started -document RIP vs install copy behavior in pickconfig

docs/src/getting-started/running-linuxcnc.adoc

@@ -151,6 +151,35 @@ for a configuration to a new subdirectory of ~/linuxcnc/configs
 created directory will include at least one INI file (iniflename.ini)
 that is used to describe a specific configuration.
 
+=== When the copy happens
+
+The Configuration Selector decides whether to copy a sample
+configuration to ~/linuxcnc/configs based on where the source
+configuration lives:
+
+* *Package install (deb, rpm, distro packages)*: sample configurations
+  live under a system path. The selector copies the configuration to
+  ~/linuxcnc/configs before running it, so edits never touch the
+  system-wide files.
+* *Run-In-Place (RIP) build*: the source tree is normally writable by
+  the user who built it, so the selector runs the configuration
+  directly from the source tree without copying. Edits made through
+  the configuration apply to the files in the RIP tree.
+
+=== Forcing a copy from a RIP build
+
+To test the copy-and-run path from a RIP build (or to keep a
+personal copy of a sample configuration outside the source tree),
+set the `debug_pickconfig` environment variable before launching
+LinuxCNC:
+
+----
+debug_pickconfig=1 linuxcnc
+----
+
+With this set, the selector copies the chosen sample configuration
+to ~/linuxcnc/configs even though the RIP source tree is writable.
+
 File resources within the copied directory will typically
 include one or more INI file (filename.ini) for related
 configurations and a tool table file (toolfilename.tbl).