lib/charset: fix compile warnings

This commit fixes the following compile warnings
for the documentation.

./include/charset.h:276: warning: Function parameter or member 'size' not described in 'u16_strlcat'
./include/charset.h:276: warning: Excess function parameter 'count' description in 'u16_strlcat'

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>