docs: shorten PHP upload limit comment

docker/librespeed-php.ini

@@ -1,18 +1,5 @@
-; LibreSpeed-recommended PHP override.
-;
-; speedtest_worker.js uploads in 20 MB chunks by default
-; (xhr_ul_blob_megabytes: 20). PHP's stock post_max_size = 8M rejects
-; every chunk: PHP emits a "POST Content-Length ... exceeds the
-; limit" startup warning and prevents empty.php from sending its
-; response headers (Cache-Control, Pragma, Connection, and CORS
-; under ?cors).
-;
-; The upload throughput number itself is unaffected — the worker
-; reads bytes-on-wire from xhr.upload.onprogress, not the response
-; body — but the response from empty.php is otherwise malformed.
-;
-; 32M is the next round number above the worker's 20M default; it
-; leaves headroom for operators who tune xhr_ul_blob_megabytes
-; upwards.
+; speedtest_worker.js defaults xhr_ul_blob_megabytes to 20.
+; PHP's stock post_max_size = 8M rejects those upload chunks before
+; empty.php can send its headers. 32M leaves modest headroom.
 
 post_max_size = 32M