bug#41981: Reference Manual on Vtables: Missing Information on Permission "h"

[dsmich]

Is is documented in the docstring:

scheme@(guile-user)> ,d make-struct-layout
- Scheme Procedure: make-struct-layout fields
     Return a new structure layout object.

     FIELDS must be a string made up of pairs of characters strung
     together.  The first character of each pair describes a field type,
     the second a field protection.  Allowed types are 'p' for
     GC-protected Scheme data, 'u' for unprotected binary data.  Allowed
     protections are 'w' for normal fields or 'h' for hidden fields.

     Hidden fields are writable, but they will not consume an
     initializer arg passed to `make-struct'.  They are useful to add
     slots to a struct in a way that preserves backward-compatibility
     with existing calls to `make-struct', especially for derived
     vtables.