vo_opengl: clarify RA_CAP_DIRECT_UPLOAD

This no longer concerns the API user except in as much as the API user
probably wants to know whether or not PBOs are active, so keep around
the CAP field even though it's mostly useless now.

video/out/opengl/ra.h

@@ -128,7 +128,8 @@ struct ra_tex_upload_params {
     // Uploading from buffer:
     struct ra_buf *buf; // Buffer to upload from (mutually exclusive with `src`)
     size_t buf_offset;  // Start of data within buffer (bytes)
-    // Uploading directly: (requires RA_CAP_DIRECT_UPLOAD)
+    // Uploading directly: (Note: If RA_CAP_DIRECT_UPLOAD is not set, then this
+    // will be internally translated to a tex_upload buffer by the RA)
     const void *src;    // Address of data
     // For 2D textures only:
     struct mp_rect *rc; // Region to upload. NULL means entire image