regulator: Fix documentation for of_node parameter of regulator_register()

Commit 5bc75a88 ("kernel-doc: fix new warning in regulator core")
added documentation for of_node to address a warning but the
documentation didn't explain what the parameter is for so would be
likely to be unhelpful for users.  Clarify that.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>