kernel: add public API doc for K_SSE_REGS

This adds public API documentation for the thread bit
K_SSE_REGS. It was previously a single line comment.

Signed-off-by: Daniel Leung <[email protected]>

Author: dcpleung

include/kernel.h

@@ -214,7 +214,15 @@ extern void k_thread_foreach_unlocked(
 /* x86 Bitmask definitions for threads user options */
 
 #if defined(CONFIG_FPU_SHARING) && defined(CONFIG_X86_SSE)
-/* thread uses SSEx (and also FP) registers */
+/**
+ * @brief FP and SSE registers are managed by context switch on x86
+ *
+ * @details
+ * This option indicates that the thread uses the x86 CPU's floating point
+ * and SSE registers. This instructs the kernel to take additional steps to
+ * save and restore the contents of these registers when scheduling
+ * the thread. No effect if @kconfig{CONFIG_X86_SSE} is not enabled.
+ */
 #define K_SSE_REGS (BIT(7))
 #endif
 #endif