bsd-user: Bring in docs from linux-user for signal_pending

This is currently unused, so no code adjustments are needed.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

bsd-user/qemu.h

@@ -86,7 +86,14 @@ typedef struct TaskState {
     struct image_info *info;
 
     struct emulated_sigtable sigtab[TARGET_NSIG];
-    int signal_pending; /* non zero if a signal may be pending */
+    /*
+     * Nonzero if process_pending_signals() needs to do something (either
+     * handle a pending signal or unblock signals).
+     * This flag is written from a signal handler so should be accessed via
+     * the qatomic_read() and qatomic_set() functions. (It is not accessed
+     * from multiple threads.)
+     */
+    int signal_pending;
 
     uint8_t stack[];
 } __attribute__((aligned(16))) TaskState;