Better travis config comments [skip ci] (#15996)

.travis.yml

@@ -148,7 +148,14 @@ install:
     fi
 
 before_script:
-  # Disable the HHVM JIT and sessions GC for faster Unit Testing
+  #
+  # Disable:
+  # 1) the HHVM JIT for faster testing;
+  # 2) the session GC for testing stability.
+  #
+  # The second allows to avoid accidental unpredictable failings with message:
+  # `ps_files_cleanup_dir: opendir(/var/lib/hhvm/sessions) failed: Permission denied (13)`
+  #
   - if [[ $TRAVIS_PHP_VERSION = hhv* ]]; then
       echo 'hhvm.jit = 0' >> /etc/hhvm/php.ini;
       echo 'session.gc_probability = 0' >> /etc/hhvm/php.ini;