Enhance comments for large dnode project

Fix a few nits in the comments from large dnodes. Also import
some of the commit message as a comment in the code, making
it more accessible.

Reviewed-by: @rottegift 
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Wilson <george.wilson@delphix.com>
Signed-off-by: Matt Ahrens <mahrens@delphix.com>
Closes #6551

module/zfs/dnode.c

@@ -1176,6 +1176,18 @@ dnode_rele_slots(dnode_children_t *children, int idx, int slots)
 }
 
 /*
+ * When the DNODE_MUST_BE_FREE flag is set, the "slots" parameter is used
+ * to ensure the hole at the specified object offset is large enough to
+ * hold the dnode being created. The slots parameter is also used to ensure
+ * a dnode does not span multiple dnode blocks. In both of these cases, if
+ * a failure occurs, ENOSPC is returned. Keep in mind, these failure cases
+ * are only possible when using DNODE_MUST_BE_FREE.
+ *
+ * If the DNODE_MUST_BE_ALLOCATED flag is set, "slots" must be 0.
+ * dnode_hold_impl() will check if the requested dnode is already consumed
+ * as an extra dnode slot by an large dnode, in which case it returns
+ * ENOENT.
+ *
  * errors:
  * EINVAL - invalid object number.
  * ENOSPC - hole too small to fulfill "slots" request