Clarify documentation about the custom init script

Author: ianfixes

REFERENCE.md

@@ -46,7 +46,7 @@ This allows a file (or glob) pattern to be executed in your tests directory, cre
 
 ### `CUSTOM_INIT_SCRIPT` environment variable
 
-If set, testing will execute (using `/bin/sh`) the script referred to by this variable -- relative to the current working directory.  This enables use cases like the GitHub action to install custom library versions (i.e. a version of a library that is different than what the library manager would automatically install by name) prior to CI test runs.
+If set, testing will execute (using `/bin/sh`) the script referred to by this variable -- relative to the current working directory (i.e. the root directory of the library).  The script will _run_ in the Arduino Libraries directory (changing to the Libraries directory, running the script, and returning to the individual library root afterward).  This enables use cases like the GitHub action to install custom library versions (i.e. a version of a library that is different than what the library manager would automatically install by name) prior to CI test runs.
 
 
 ### `USE_SUBDIR` environment variable