binfmt_misc: add F option description to documentation

            This feature should be used with care as the interpreter

            will run with root permissions when a setuid binary owned by root

            is run with binfmt_misc.

      'F' - fix binary.  The usual behaviour of binfmt_misc is to spawn the

      	    binary lazily when the misc format file is invoked.  However,

	    this doesn't work very well in the face of mount namespaces and

	    changeroots, so the F mode opens the binary as soon as the

	    emulation is installed and uses the opened image to spawn the

	    emulator, meaning it is always available once installed,

	    regardless of how the environment changes.

There are some restrictions: