docs: PORTING: Document the remaining required flash APIs

[danielriege]

What

Adds three functions to the list of flash APIs a system must provide in
docs/PORTING.md, and a note on the offset semantics of one of them.

Why

The document's "main, also required, set of API functions" is missing three
functions that common code in boot/bootutil calls unconditionally, and that
no implementation outside a port supplies:

Function	Call sites in boot/bootutil
flash_area_get_sector()	8
flash_device_base()	2
flash_area_id_from_image_slot()	2

All three are declared in flash_map_backend.h and implemented per port (see
boot/zephyr/flash_map_extended.c); none has a definition in boot/bootutil.
A port written against the current document therefore fails to link, and the
document gives no hint as to what is missing.

flash_area_get_sector() is worth more than a one-line entry, because it is
easy to implement in a way that links and then misbehaves: both the off
parameter and the fs_off written back are relative to the flash area, not
to the device. An implementation that treats them as device-absolute — which is
a natural first guess — leaves the sector walk in boot_erase_region() unable
to reach its termination condition. The added note states the convention and
points at the Zephyr implementation as a reference.

Notes

• Documentation only; no code or behavioural change.
• No release note stub, per docs/contributing.md ("Release notes are generally
  not needed for: Some documentation improvements").
• flash_area_id_to_image_slot() was deliberately not added: it has no
  call sites in boot/bootutil and is not declared in flash_map_backend.h,
  so it does not appear to be required of a port.