^X feedback when held by unseen monster

When swallowed and blind, the swallowing monster is described
accurately, but being held rather than swallowed describes the
monster as "it".  That's normal, but the status feedback section
of ^X output lists
|You are held by it.
which looks pretty weird.  Change that to be
|You are held by an unseen creature.

include/mextra.h

@@ -18,7 +18,8 @@
  *       2. Add a pointer to your new struct to the mextra struct in this
  *          file.
  *       3. Add a referencing macro at bottom of this file after the mextra
- *          struct (see MGIVENNAME, EGD, EPRI, ESHK, EMIN, or EDOG for examples).
+ *          struct (see MGIVENNAME, EGD, EPRI, ESHK, EMIN, or EDOG for
+ *          examples).
  *       4. If your new field isn't a pointer and requires a non-zero value
  *          on initialization, add code to init_mextra() in src/makemon.c
  *       5. Create a newXX(mtmp) function and possibly a free_XX(mtmp)