client API: comment about signal handlers

Sharing a process sure is hard in POSIX.

The rationale is that you'd have to handle EINTR on every single
blocking syscall. stream_file.c does not seem to handle it on read()
calls.

It appears that on most modern systems, this can happen only if you call
sigaction(), and incompetently forget to add SA_RESTART. signal()
usually adds it.

libmpv/client.h

@@ -147,6 +147,8 @@ extern "C" {
  * - mpv may start sub processes, so overriding SIGCHLD, or waiting on all PIDs
  *   (such as calling wait()) by the parent process or any other library within
  *   the process must be avoided. libmpv itself only waits for its own PIDs.
+ * - If anything in the process registers signal handlers, they must set the
+ *   SA_RESTART flag. Otherwise you WILL get random failures on signals.
  *
  * Encoding of filenames
  * ---------------------