RFA: document raw/virtual stuff

[Eli Zaretskii]

> + @section Using Different Register and Memory Data Representations
> + 
> + Some architectures use one representation for a value when it lives in a
> + register, but use a different representation when it lives in memory.
> + In GDB's terminology, the @dfn{raw} representation is the one used in
> + the target registers, and the @dfn{virtual} representation is the one
> + used in memory, and within GDB @code{struct value} objects.

I suggest these index entries for the above paragraph:

  @cindex raw representation
  @cindex virtual representation
  @cindex @code{struct value}, raw and virtual representation