Clarify the proscription of $<

Clarify the proscription of $<

Clarify the situations in which $< can be used, as we do use it in
some places in the tree. It is sufficiently portable in pattern rules,
but should not be used elsewhere

Also, change the function definition example to match the format used
through out the source tree.

Reviewed-on: http://gerrit.openafs.org/http://gerrit.openafs.org/160
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>