firewire: core: document fw_csr_string's truncation of long strings

fw_csr_string() truncates and terminates target strings like strlcpy()
does.  Unlike strlcpy(), it returns the target strlen, not the source
strlen, hence users of fw_csr_string() are unable to detect truncation.

Point this behavior out in the kerneldoc comment.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>

drivers/firewire/core-device.c

@@ -115,6 +115,9 @@ static int textual_leaf_to_string(const u32 *block, char *buf, size_t size)
  *
  * The string is taken from a minimal ASCII text descriptor leaf after
  * the immediate entry with @key.  The string is zero-terminated.
+ * An overlong string is silently truncated such that it and the
+ * zero byte fit into @size.
+ *
  * Returns strlen(buf) or a negative error code.
  */
 int fw_csr_string(const u32 *directory, int key, char *buf, size_t size)