to_hdf(mode='a') overwrites HDF table, does not append
This seems to be the same issue as #4584 (https://github.com/pandas-dev/pandas/issues/4584), but the issue seems to persist.

I'm using Pandas 0.19.1. The docs state that for to_hdf(), "mode = 'a' " means "Append; an existing file is opened for reading and writing"
http://pandas.pydata.org/pandas-docs/version/0.19.1/generated/pandas.DataFrame.to_hdf.html

But the following code example shows to_hdf(... mode='a') is overwriting, not appending 

```
df1 = pd.DataFrame(np.random.randint(0,100,size=(100, 4)), columns=list('ABCD'))
df2 = pd.DataFrame(np.random.randint(0,100,size=(100, 4)), columns=list('ABCD'))
df3 = pd.DataFrame(np.random.randint(0,100,size=(100, 4)), columns=list('ABCD'))
df4 = pd.DataFrame(np.random.randint(0,100,size=(100, 4)), columns=list('ABCD'))

for df in (df1, df2, df3, df4):
    df.to_hdf('example.h5', 'my_hdf_table', mode='a', format='table')    

store = pd.HDFStore('example.h5')
print(store.items)    
#prints "/my_hdf_table             frame_table  (typ->appendable,nrows->100,..."

```

However, using `df.to_hdf('example.h5', 'my_hdf_table2', format='table', append=True)`  does result in appending, not overwriting. 

Maybe this is a documentation issue not a code issue? Why do we have "mode = a" described as "append" if a separate non-default option "append = True" is still required for appending?

As always, thanks for all the work on a fantastic library.