Add a FS::check() optional method (#6340)

* Add a FS::check() optional method

Fixes #2634

Expose any low-level filesystem check operations for users, and add
documentation on this and the gc() methods.

* Update doc w/more gc() info and link

doc/filesystem.rst

@@ -402,6 +402,31 @@ block size - ``pageSize`` — filesystem logical page size - ``maxOpenFiles``
 ``maxPathLength`` — max file name length (including one byte for zero
 termination)
 
+gc
+~~
+
+.. code:: cpp
+
+    SPIFFS.gc()
+
+Only implemented in SPIFFS.  Performs a quick garbage collection operation on SPIFFS,
+possibly making writes perform faster/better in the future.  On very full or very fragmented
+filesystems, using this call can avoid or reduce issues where SPIFFS reports free space
+but is unable to write additional data to a file.  See `this discussion
+<https://github.com/esp8266/Arduino/pull/6340#discussion_r307042268>` for more info.
+
+check
+~~~~~
+
+.. code:: cpp
+
+    SPIFFS.begin();
+    SPIFFS.check();
+
+Only implemented in SPIFFS.  Performs an in-depth check of the filesystem metadata and
+correct what is repairable.  Not normally needed, and not guaranteed to actually fix
+anything should there be corruption.
+
 Directory object (Dir)
 ----------------------