strlcpy/strlcat: Remove references to snprintf

While strlcpy and snprintf are somewhat similar, there's big differences
between strlcat and snprintf which leads to confusion. Remove the
comparison, since it's ultimately not that useful: the snprintf man page
has similar language to strlcpy, so it doesn't provide a better
reference. The two implementations are otherwise unrelated.

Reviewed by: bcr
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D27228