lib: charset: correct utf8_utf16_strnlen() description

Correct the description of utf8_utf16_strnlen() and utf8_utf16_strlen() to
reflect that they return u16 count and not byte count.

For these functions and utf16_utf8_strnlen() describe the handling of
invalid code sequences.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>