doc: Correct context description for some JIT support GUCs

doc: Correct context description for some JIT support GUCs

The documentation for jit_debugging_support and jit_profiling_support
previously stated that these parameters can only be set at server start.

However, both parameters use the PGC_SU_BACKEND context, meaning they
can be set at session start by superusers or users granted the appropriate
SET privilege, but cannot be changed within an active session.

This commit updates the documentation to reflect the actual behavior.

Backpatch to all supported versions.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAHGQGwEpMDpB-K8SSUVRRHg6L6z3pLAkekd9aviOS=ns0EC=+Q@mail.gmail.com
Backpatch-through: 14