fmt: Adjust stack allocated buffer, add comments

Adjusted buffer size for u64 case to fit the largest 64 bit decimal
number, added comments on all decimal buffers to explain which number
will fit.

sys/fmt/fmt.c

@@ -414,14 +414,14 @@ void print(const char *s, size_t n)
 
 void print_u32_dec(uint32_t val)
 {
-    char buf[10];
+    char buf[10]; /* "4294967295" */
     size_t len = fmt_u32_dec(buf, val);
     print(buf, len);
 }
 
 void print_s32_dec(int32_t val)
 {
-    char buf[11];
+    char buf[11]; /* "-2147483648" */
     size_t len = fmt_s32_dec(buf, val);
     print(buf, len);
 }
@@ -448,7 +448,7 @@ void print_u64_hex(uint64_t val)
 
 void print_u64_dec(uint64_t val)
 {
-    char buf[18];
+    char buf[20]; /* "18446744073709551615" */
     size_t len = fmt_u64_dec(buf, val);
     print(buf, len);
 }