DOC: df.iterrows() returns view for homogenous columns, and copy for heterogenous / perf warning
perf discussion: http://stackoverflow.com/questions/24870953/does-iterrows-have-performance-issues/24871316#24871316

```
import pandas as pd
df = pd.DataFrame({'a':range(5),'b':range(5)})
print df

for _, row in df.iterrows():
    row.a += 1
print df

df['c'] = 'what'

for _, row in df.iterrows():
    row.b += 1
print df

   a  b
0  0  0
1  1  1
2  2  2
3  3  3
4  4  4

[5 rows x 2 columns]
   a  b
0  1  0
1  2  1
2  3  2
3  4  3
4  5  4

[5 rows x 2 columns]
   a  b     c
0  1  0  what
1  2  1  what
2  3  2  what
3  4  3  what
4  5  4  what

[5 rows x 3 columns]
```
