Make tests work reliably on the Pi

While the tests work well on a PC or Travis, the Pi (where I ought to be
running them!) has some issues with the timing tests. Need to relax the
tolerance of the "assert_states_and_times" method to 0.05 seconds
otherwise it periodically fails even on something reasonably quick like
a Pi 2 (less failures on a Pi 3 but still occasionally).

Also reduced default fps to 25; if the default timing occasionally fails
on a Pi 2 it's evidently too fast for a Pi 1 and shouldn't be the
default; 25 also doesn't look any different to me on a pulsing LED.

There's also a bunch of miscellaneous fixes in here; last minute typos
and chart re-gens for the 1.2 release.

gpiozero/mixins.py

@@ -374,7 +374,7 @@ class HoldMixin(EventsMixin):
     @hold_time.setter
     def hold_time(self, value):
         if value < 0:
-            raise BadWaitTime('source_delay must be 0 or greater')
+            raise BadWaitTime('hold_time must be 0 or greater')
         self._hold_time = float(value)
 
     @property
@@ -403,8 +403,8 @@ class HoldMixin(EventsMixin):
         The length of time (in seconds) that the device has been held for.
         This is counted from the first execution of the :attr:`when_held` event
         rather than when the device activated, in contrast to
-        :attr:`active_time`. If the device is not currently held, this is
-        ``None``.
+        :attr:`~EventsMixin.active_time`. If the device is not currently held,
+        this is ``None``.
         """
         if self._held_from is not None:
             return time() - self._held_from