doc: migration guide: posix_time.h, posix_signal.h, posix_limits.h

Add notes in the migration guide that users should no longer include
`<zephyr/posix/time.h>` or `<zephyr/posix/signal.h>`, and to include
them in the standard paths, along with `<limits.h>`.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

Author: cfriedt

doc/releases/migration-guide-4.3.rst

@@ -41,6 +41,16 @@ Base Libraries
 * ``Z_MIN``, ``Z_MAX`` and ``Z_CLAMP`` macros have been renamed to
   :c:macro:`min` :c:macro:`max` and :c:macro:`clamp`.
 
+* The header files ``<zephyr/posix/time.h>``, ``<zephyr/posix/signal.h>`` should no longer be used.
+  Include them in the standard path as ``<time.h>``, and ``<signal.h>``, provided by the C library.
+  Non-POSIX C library maintainers may include :zephyr_file:`include/zephyr/posix/posix_time.h`
+  and :zephyr_file:`include/zephyr/posix/posix_signal.h` to portably provide POSIX definitions.
+
+* POSIX limits are no longer defined in ``<zephyr/posix/posix_features.h>``. Similarly, include them
+  in the standard path via ``<limits.h>``, provided by the C library. Non-POSIX C library maintainers
+  may include :zephyr_file:`include/zephyr/posix/posix_limits.h` for Zephyr's definitions. Some
+  runtime-invariant values may need to be queried via :c:func:`sysconf`.
+
 Boards
 ******