kgdb: update outdated references to kgdb_wait()

kgdb: update outdated references to kgdb_wait()

The function kgdb_wait() was folded into the static function
kgdb_cpu_enter() by commit 62fae312197a ("kgdb: eliminate
kgdb_wait(), all cpus enter the same way").  Update the four stale
references accordingly:

 - include/linux/kgdb.h and arch/x86/kernel/kgdb.c: the
   kgdb_roundup_cpus() kdoc describes what other CPUs are rounded up
   to call.  Because kgdb_cpu_enter() is static, the correct public
   entry point is kgdb_handle_exception(); also fix a pre-existing
   grammar error ("get them be" -> "get them into") and reflow the
   text.

 - kernel/debug/debug_core.c: replace with the generic description
   "the debug trap handler", since the actual entry path is
   architecture-specific.

 - kernel/debug/gdbstub.c: kgdb_cpu_enter() is correct here (it
   describes internal state, not a call target); add the missing
   parentheses.

Suggested-by: Daniel Thompson <daniel@riscstar.com>
Assisted-by: unnamed:deepseek-v3.2 coccinelle
Signed-off-by: Kexin Sun <kexinsun@smail.nju.edu.cn>