document issues with reentrancy in trio guest mode

Some host loops like to "reenter" the loop, e.g. in Qt stuff like QDialog.exec, a bunch of utility functions on QFileDialog, QMenu.exec, etc. See also:

• https://www.qt.io/blog/2010/02/23/unpredictable-exec
• https://stackoverflow.com/questions/35561182/why-should-nesting-of-qeventloops-be-avoided

Need to document what happens if you use these from inside a Trio function (not great – does it merely block Trio, or is it catastrophic? at first I thought catastrophic, but maybe it's merely the same as doing any blocking operation), and what to do instead (use alternate APIs, or push them out to run in the host event loop directly and wait for the result from Trio).

Start with Trio's guest mode documentation and the linked Qt references about nested event loops. Establish whether reentrant host-loop APIs block or disrupt Trio, then document the observed behavior and supported alternatives for invoking them from Trio.