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>