math.h: amend abs() kernel-doc and add a note about signed type limits

- amend the kernel-doc so the description is decoupled from the
parameter descriptions.

- add a note to explain behaviour for the signed types when supplied
value is the minimum (e.g., INT_MIN for int type).

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Andy Shevchenko <[e-mail]>
Cc: Jonathan Cameron <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/math.h

···
 
 /**
  * abs - return absolute value of an argument
- * @x: the value.  If it is unsigned type, it is converted to signed type first.
- *     char is treated as if it was signed (regardless of whether it really is)
- *     but the macro's return type is preserved as char.
+ * @x: the value.
  *
- * Return: an absolute value of x.
+ * If it is unsigned type, @x is converted to signed type first.
+ * char is treated as if it was signed (regardless of whether it really is)
+ * but the macro's return type is preserved as char.
+ *
+ * NOTE, for signed type if @x is the minimum, the returned result is undefined
+ * as there is not enough bits to represent it as a positive number.
+ *
+ * Return: an absolute value of @x.
  */
 #define abs(x)	__abs_choose_expr(x, long long,				\
 		__abs_choose_expr(x, long,				\