Improve docs of more system calls

This change also found a few POSIX compliance bugs with errnos. Another
bug was discovered where, on Windows, pread() and pwrite() could modify
the file position in cases where ReadFile() returned an error e.g. when
seeking past the end of file. We also have more tests!

libc/calls/clock_getres.c

@@ -17,10 +17,10 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/asan.internal.h"
-#include "libc/intrin/strace.internal.h"
 #include "libc/calls/struct/timespec.internal.h"
 #include "libc/dce.h"
 #include "libc/intrin/describeflags.internal.h"
+#include "libc/intrin/strace.internal.h"
 #include "libc/sysv/consts/clock.h"
 #include "libc/sysv/errfuns.h"
 #include "libc/time/time.h"
@@ -49,6 +49,12 @@ static int sys_clock_getres_xnu(int clock, struct timespec *ts) {
 
 /**
  * Returns granularity of clock.
+ *
+ * @return 0 on success, or -1 w/ errno
+ * @error EPERM if pledge() is in play without stdio promise
+ * @error EINVAL if `clock` isn't supported on this system
+ * @error EFAULT if `ts` points to bad memory
+ * @threadsafe
  */
 int clock_getres(int clock, struct timespec *ts) {
   int rc;