Sanitize input for plotItem.showGrid (was documentation fix for plotItem.showGrid alpha setting)

[NilsNemitz]

Looking through some issues, I came across #1484:

setting plotItem.showGrid(True, True, alpha=0.5) raises TypeError: setValue(self, int): argument 1 has unexpected type 'numpy.float64'

This is a documentation error:
The expected alpha value does not range from 0.0 to 1.0 (as in the current docstring), but instead ranges from 0-255.

This PR updates the docstring to show that and adds a cast to int to avoid raising an error.
The alpha=0.5 value in the original report will no longer result in an error and draw an invisible, fully transparent grid.
Setting alpha=127 creates the desired half-transparent grid.

So this should close #1484.

[ixjlyons]

I don't think this is a documentation error actually. The slider goes from 0 to 255:

pyqtgraph/pyqtgraph/graphicsItems/PlotItem/plotConfigTemplate.ui

Lines 298 to 300 in e29f865

	<widget class="QSlider" name="gridAlphaSlider">
	<property name="maximum">
	<number>255</number>

So using alpha as a [0, 1] scaler for the max of the slider is doing what's intended, it's just that QSlider.setValue takes an int type. setValue(int(v)) should suffice.

edit: Also I should point out the proposed fix would be a change in behavior, and I actually don't see the error in #1484 -- I'm not sure if there's a platform-specific thing going on with type strictness or what

[NilsNemitz]

True.

@ixjlyons , this update restores the 0-1 range and just adds the cast to int.

[ixjlyons]

Looks good to me. The comment there is helpful too 👍

[j9ac9k]

LGTM, merging. Thanks @NilsNemitz !