i2c: add kdoc for the new debugfs entry of clients

When adding the new debugfs entry, its kdoc equivalent was forgotten.
Add it now.

Fixes: d06905d6 ("i2c: add core-managed per-client directory in debugfs")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/r/20250115163146.6c48f066@canb.auug.org.au


Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>