libc: Document errno.h

libs/libc/include/errno.h

@@ -1,15 +1,16 @@
 #ifndef _ERRNO_H
 #define _ERRNO_H
 
+/* The last error encountered during a call to a library or system function. */
 extern int errno;
 
-#define EPERM 1
-#define ENOENT 2
-#define EBADF 9
-#define ENOMEM 12
-#define EISDIR 21
-#define EINVAL 22
-#define EMFILE 24
-#define ENOSYS 38
+#define EPERM 1   // Operation not permitted
+#define ENOENT 2  // No such file or directory
+#define EBADF 9   // Bad file descriptor
+#define ENOMEM 12 // Cannot allocate memory
+#define EISDIR 21 // Is a directory
+#define EINVAL 22 // Invalid argument
+#define EMFILE 24 // Too many open files
+#define ENOSYS 38 // Function not implemented
 
 #endif