of: property: Fix documentation for out values

Property fetching functions which return number of successfully fetched
properties should not state that out-values are only modified if 0 is
returned. Fix this. Also, "pointer to return value" is slightly suboptimal
phrase as "return value" commonly refers to value function returns (not via
arguments). Rather use "pointer to found values".

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Reviewed-by: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>