style: Document recent Go-pointer exceptions

The general rule seems to be:

  If Go's default value has the same semantics we'd use for an unset
  value, don't bother with a pointer.

I'm not sure how well that squares with [1]:

  We want a consistent way to identify unset settings.

But if the falsy values count as "unset", maybe the "null is a
consistent identifier for unset" approach was never really viable.

I'm also not sure if the new style extends to integers where zero has
the same semantics as unset values.  It sounds like Michael was ok
with no pointers for those values [2], but OOMScoreAdj (where zero
clearly means "do nothing") got a pointer in #233 [3].  More clarity
on the threshold would be nice.

[1]: #233 (comment)
[2]: #233 (comment)
[3]: https://github.com/opencontainers/specs/pull/233/files#diff-34c30be66233f08b447fb608ea0e66bbR206

Signed-off-by: W. Trevor King <wking@tremily.us>

style.md

@@ -13,9 +13,12 @@ The redundancy reduction from removing the namespacing prefix is not useful enou
 ## Optional settings should have pointer Go types
 
 So we have a consistent way to identify unset values ([source][optional-pointer]).
+The exceptions are slices, maps, and booleans where the default should be false, in which case `omitempty` is sufficient and no pointer is needed ([source][no-pointer-for-slices] and [source][no-pointer-for-boolean]).
 
 [capabilities]: config-linux.md#capabilities
 [class-id]: runtime-config-linux.md#network
 [integer-over-hex]: https://github.com/opencontainers/specs/pull/267#discussion_r48360013
 [keep-prefix]: https://github.com/opencontainers/specs/pull/159#issuecomment-138728337
+[no-pointer-for-boolean]: https://github.com/opencontainers/specs/pull/290#discussion_r50296396
+[no-pointer-for-slices]: https://github.com/opencontainers/specs/pull/316/files#r50782982
 [optional-pointer]: https://github.com/opencontainers/specs/pull/233#discussion_r47829711