Bug report

Bug summary

By using GridSpec update method I am unable to update width_ratios argument. The GridSpec.update method does not clearly document which keywords argument it takes.

Code for reproduction

import matplotlib.gridspec as gridspec

gs1 = gridspec.GridSpec(1, 3, width_ratios=[3,1,2], left =0.1, right=0.2, wspace=0.05)

Now look into documentation: gs1.update?:

gs1.update(**kwargs)
Docstring:
Update the current values.  If any kwarg is None, default to
the current value, if set, otherwise to rc.

Well, why not updating width_ratios, which is a keyword argument:

gs1.update(width_ratios=[3,1,2], )

Actual outcome
Error:

/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/matplotlib/gridspec.py in update(self, **kwargs)
    228                 setattr(self, k, v)
    229             else:
--> 230                 raise AttributeError("%s is unknown keyword" % (k,))
    231 
    232 

AttributeError: width_ratios is unknown keyword

Expected outcome

• documentation describing which arguments are taken
• possibility to adjust the width and height ratios.

Matplotlib version

• Matplotlib version,
  '1.5.3'

• Python version
  3.5

• and Platform (Windows, OSX, Linux ...)
  OSX

• How did you install Matplotlib and Python (pip, anaconda, from source ...)
  pip3