docs: say why openlara reads nothing it is given

OpenLara asks for the system directory to keep its own shader and cache
files under openlara/cache, which it writes and reads back itself.

emulators/openlara.yml

@@ -10,6 +10,12 @@ display_name: "Tomb Raider (OpenLara)"
 cores: [openlara]
 systems: [tomb-raider]
 verification: existence
+exclusion_note: >
+  The system directory holds openlara/cache, which the core creates
+  (src/platform/libretro/main.cpp:177-196) and fills with the shader program
+  binaries and cache streams it writes itself, read back only as a cache
+  (src/gapi/gl.h:810-816, src/utils.h:2361-2367). Nothing in it is supplied by
+  the user.
 notes: >
   Open-source reimplementation of the classic Tomb Raider engine (TR1-TR4).
   Accepts phd|psx|tr2|sat extensions (src/platform/libretro/main.cpp:246). need_fullpath is true -