guestfs_find: Document protocol limits for this API call.

guestfs_find has to send the complete list of files in a single
protocol message (hence, limited to 2-4 MB).  Unfortunately on
a typical Linux guest, guestfs_find ("/") will exceed this limit
resulting in an error.

Therefore we should add an unlimited version of this call in a
future commit.  This commit just documents the current limit.