Update documentation for folly::Subprocess::pollUnchecked

Summary:
Previously it was stated that this method would certainly call `CalledProcessError` if the process has exited. However, for instances where the poll does not go through due to the process already having exited, this is not the case and the function will throw a std::logic_error. To avoid confusion here, I have added clarifications on the exact behaviour for future travellers.

Context: https://fb.workplace.com/groups/560979627394613/permalink/3546349242190955/

Reviewed By: dmm-fb

Differential Revision: D91219516

fbshipit-source-id: 2d8a18b16a4d71c65ae8b319fe873d6fb5b24783

Author: Ollie Cole

folly/Subprocess.h

@@ -659,6 +659,10 @@ class Subprocess {
    * Poll the child's status.  If the process is still running, return false.
    * Otherwise, return true if the process exited with status 0 (success),
    * or throw CalledProcessError if the process exited with a non-zero status.
+   * Note: this should only be called for processes in the RUNNING state. If
+   * another trigger has caused the process to register as exited, or the
+   * process is yet to start before this is called, this will throw a
+   * std::logic_error.
    */
   bool pollChecked();