Log Message:
mail/thunderbird: Add comment about excessive WRKDIR space requirement

This is just a comment; perhaps in the future there might be a
mechanism to store this information, probably in a database vs source,
so that people who are using WRKOBJDIR in a tmpfs can repoint it for
the few packages that use unreasonable amounts of space.  (The binary
package is only 200 MB, so it's hard to explain 22GB, and
similarly-sized firefox doesn't have this problem.)