docs: clarify prog.full_path even more

The previous description update was lacking an example of why
external_program cares about inter-target dependencies.

docs/yaml/objects/external_program.yaml

@@ -17,9 +17,10 @@ methods:
 
     **NOTE:** You should not usually need to use this method. Passing the
     object itself should work in most contexts where a program can appear,
-    and allows Meson to setup inter-target dependencies correctly. Only
-    use this if you specifically need a string, such as when embedding a
-    program path into a header file.
+    and allows Meson to setup inter-target dependencies correctly (for
+    example in cases where a program might be overridden by a [[build_tgt]]).
+    Only use this if you specifically need a string, such as when embedding
+    a program path into a header file.
 
     For example:
 
@@ -43,9 +44,10 @@ methods:
 
     **NOTE:** You should not usually need to use this method. Passing the
     object itself should work in most contexts where a program can appear,
-    and allows Meson to setup inter-target dependencies correctly. Only
-    use this if you specifically need a string, such as when embedding a
-    program path into a header file.
+    and allows Meson to setup inter-target dependencies correctly (for
+    example in cases where a program might be overridden by a [[build_tgt]]).
+    Only use this if you specifically need a string, such as when embedding
+    a program path into a header file.
 
     For example: