i2c: document new callbacks in i2c_algorithm

When updating the callbacks, adding their kernel-doc was forgotten. Add
it now.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/r/20240712165527.75e4ddc9@canb.auug.org.au
Fixes: a93c2e5f

 ("i2c: reword i2c_algorithm according to newest specification")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>