doc: release-notes: document aarch32 cortex_a_r linker command file changes

Document the changes implemented in #60368:
* Placement of the .bss and .noinit sections at the end of the binary
  so that large zero-/uninitialized data structures such as heaps,
  arrays etc. don't have to be padded in the resulting binary.
* Location of the z_mapped_start marker: prevents the assignment
  of the single 4k-page wide .vectors section right at the RAM
  base address as dynamic memory by the MMU at run-time. Instead
  of pointing to the start of the subsequent .text section, the
  z_mapped_start marker now covers all the data contained within
  the binary that ends up in RAM.

Signed-off-by: Immo Birnbaum <mail@birnbaum.immo>