Document the value of Booleans (#109)

Author: gnzlbg

riscv-elf.md

@@ -300,12 +300,13 @@ the imaginary part.
 A future version of this specification may define an ILP32 ABI for
 RV64G, but currently this is not a supported operating mode.
 
+Booleans (`bool`/`_Bool`) stored in memory or when being passed as scalar
+arguments are either `0` (`false`) or `1` (`true`).
 
 A null pointer (for all types) has the value zero.
 
 The value of `_Alignof(max_align_t)` is 16.
 
-
 ## <a name=va-list-va-start-and-va-arg></a> va_list, va_start, and va_arg
 
 The `va_list` type is `void*`. A callee with variadic arguments is responsible