* If esp_vfs_spiffs_register is called with an explicit partition
label, other SPIFFS functions (info, format, unregister) must be
called with the same label.
* On the other hand, if label was NULL in the call to
esp_vfs_spiffs_register and the first matching partition was used,
calls to the rest of the SPIFFS functions should be done with NULL
partition_label argument.
Fix the Doxygen documentation. Update the example accordingly, in case
a user modifies "partition_label" value in esp_vfs_spiffs_conf_t conf
initializer.
Closes https://github.com/espressif/esp-idf/issues/4450
