Add a comment mentioning that FilesystemCache can be used when APCu isn't available

[shish]

Add a comment mentioning that FilesystemCache can be used when APCu isn't available

APCu isn't enabled by default, which makes the example crash, and there are no hints as to what the user is supposed to do about it

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.29%. Comparing base (53f9d49) to head (87d2e68).
Report is 69 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #662      +/-   ##
============================================
- Coverage     95.72%   95.29%   -0.44%     
- Complexity     1773     1818      +45     
============================================
  Files           154      171      +17     
  Lines          4586     4841     +255     
============================================
+ Hits           4390     4613     +223     
- Misses          196      228      +32     

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[oojacoboo]

I think this one could be updated to be a code comment. But APCu should be encouraged for performance purposes. People shouldn't assume that the filesystem is suitable for cache. It really shouldn't be used.

So, it should stay as APCu with a comment that the Filesystem can be used for testing, but isn't encouraged for performance reasons!