fixup some AV API pod descriptions.

fixup some AV API pod descriptions.

In particular:

* improve  some of the "perl equivalent" entries; for example
  av_store() is *not* like $myarray[$key] = $val, since it replaces the
  stored SV with a different SV, rather than just updating the current
  SV's value.

* Also change the "perl equivalent" variable names to match the function
  parameter names, e.g. $key rather than $idx.

* Don't use 'delete' as a perl equivalent, since delete is discouraged on
  arrays.

* You don't *have* to use av_store() to change undef values inserted by
  av_unshift; e.g. you could do av_fetch() then modify the returned
  undef SV; so just delete that sentence