Improve target.h & target_ops & xfer_partial descriptions

Improve target.h & target_ops & xfer_partial descriptions

Working backwards in terms of motivation for the patch:

- When accessing memory via the xfer_partial interface, the process
  that we're accessing is indicated by inferior_ptid.  This can be
  either the same process as current inferior, or a fork child which
  does not exist in the inferior list.  This is not documented
  currently.  This commit fixes that.

- For target delegation to work, we must always make the inferior we
  want to call the target method on, the current inferior.  This
  wasn't documented, AFAICT, so this commit fixes that too.  I put
  that in the intro comment to target_ops.

- I actually started writing a larger intro comment to target_ops, as
  there was seemingly none, which I did find odd.  However, I then
  noticed the description closer to the top of the file.  I missed it
  the first time, because for some reason, that intro comment is no
  longer at the top of the file, as #includes etc. have been added
  above it over the years.  This commit fixes that too, by moving that
  intro comment to the top.

Change-Id: Id21f5462947f2a0f6f3ac0c42532df62ba355914
Approved-By: Simon Marchi <simon.marchi@efficios.com>
Approved-By: Tom Tromey <tom@tromey.com>