add: spiffs issues documentation

Issues documented:
 - SPIFFS is unable to utilize 100% of partition space
 - Long time between write function calls
 - Memory leaks when deleting file

JIRA IDF-3776

docs/en/api-reference/storage/spiffs.rst

@@ -14,6 +14,9 @@ Notes
  - Currently, SPIFFS does not support directories, it produces a flat structure. If SPIFFS is mounted under ``/spiffs``, then creating a file with the path ``/spiffs/tmp/myfile.txt`` will create a file called ``/tmp/myfile.txt`` in SPIFFS, instead of ``myfile.txt`` in the directory ``/spiffs/tmp``.
  - It is not a real-time stack. One write operation might take much longer than another.
  - For now, it does not detect or handle bad blocks.
+ - SPIFFS is able to reliably utilize only around 75% of assigned partition space.
+ - When the filesystem is running out of space, the garbage collector is trying to find free space by scanning the filesystem multiple times, which can take up to a few seconds per write function call, depending on required space.
+ - Deleting files does not always delete the whole file, which leaves unusable sections throughout the filesystem.
 
 Tools
 -----