generic: document that _c_boolean_expr_()/_c_unlikely_()/_c_likely_() are async-signal-safe

They are very fundamental. It's important that we can use them at all places.
Document it.

Author: thom311

src/c-stdaux-generic.h

@@ -129,6 +129,8 @@ extern "C" {
  *
  * Outside of macros, this has no added value.
  *
+ * This macro is async-signal-safe, provided that the condition is.
+ *
  * Return: Evaluates to the value of ``!!_x``.
  */
 #define _c_boolean_expr_(_x) _c_internal_boolean_expr_(__COUNTER__, _x)
@@ -164,6 +166,8 @@ extern "C" {
  *
  * Alias for ``__builtin_expect(!!(_x), 1)``.
  *
+ * This macro is async-signal-safe, provided that the condition is.
+ *
  * Return: The expression ``!!_x`` is evaluated and returned.
  */
 #define _c_likely_(_x) _c_internal_likely_(_x)
@@ -200,6 +204,8 @@ extern "C" {
  *
  * Alias for ``__builtin_expect(!!(_x), 0)``.
  *
+ * This macro is async-signal-safe, provided that the condition is.
+ *
  * Return: The expression ``!!_x`` is evaluated and returned.
  */
 #define _c_unlikely_(_x) _c_internal_unlikely_(_x)