making file cache store actually be able to make use of preventfilelocking config option

added comments explaining some of the detrimental effects that can have
additional comments
Revert "making file cache store actually be able to make use of preventfilelocking config option"

This reverts commit 239bbb466cf8e35fe18720380a7a75ffc3169750.
Merge branch 'patches/no-flock-file-cache' into MOODLE_26_STABLE-local