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/pread.c

@@ -32,7 +32,9 @@
 #include "libc/zipos/zipos.internal.h"
 
 /**
- * Reads from file at offset, thus avoiding superfluous lseek().
+ * Reads from file at offset.
+ *
+ * This function never changes the current position of `fd`.
  *
  * @param fd is something open()'d earlier, noting pipes might not work
  * @param buf is copied into, cf. copy_file_range(), sendfile(), etc.
@@ -40,8 +42,14 @@
  * @param offset is bytes from start of file at which read begins
  * @return [1..size] bytes on success, 0 on EOF, or -1 w/ errno; with
  *     exception of size==0, in which case return zero means no error
+ * @raise ESPIPE if `fd` isn't seekable
+ * @raise EINVAL if `offset` is negative
+ * @raise EBADF if `fd` isn't an open file descriptor
+ * @raise EIO if a complicated i/o error happened
+ * @raise EINTR if signal was delivered instead
  * @see pwrite(), write()
  * @asyncsignalsafe
+ * @threadsafe
  * @vforksafe
  */
 ssize_t pread(int fd, void *buf, size_t size, int64_t offset) {