Improve(?) explanation of SettingWithCopy warning
Improve(?) explanation of SettingWithCopy warning

After playing with R a bunch, I started feeling like the explanation of
`SettingWithCopy` wasn't getting to the core of the matter, which is actually
an essential consequence of python slice assignment semantics. Here's how
python handles chained assignment:

```python
df['foo']['bar'] = quux
df.__getitem__('foo').__setitem__('bar', quux)
```

whereas in R, it's this:

```R
df["foo"]["bar"] <- quux
df["foo"] <- `[<-`(df["foo"], "bar", quux)
df <- `[<-`(df, "foo", `[<-`(`[`(df, "foo"), "bar", quux))
```

That last is a lot of line noise, though the R method names `` `[` `` and
`` `[<-` `` are more concise than `__getitem__` and `__setitem__`! But imagine
that you could call `__setitem__` with a kwarg `inplace=False` that would cause
it to return a modified copy instead of modifying the original object. Then the
R version would translate to this in python:

```python
df = df.__setitem__('foo',
                    df.__getitem__('foo')
                      .__setitem__('bar', quux, inplace=False),
                    inplace=False)
```
This is incredibly awkward, but it has the advantage of making
`SettingWithCopy` unnecessary&mdash; *everything* is a copy, and yet things get
set nonetheless.

So this commit is an attempt to explain this without requiring the reader to
know R.

jmmease/pandas,toobaz/pandas,jorisvandenbossche/pandas,DGrady/pandas,kdebrab/pandas,harisbal/pandas,toobaz/pandas,winklerand/pandas,jorisvandenbossche/pandas,pratapvardhan/pandas,DGrady/pandas,kdebrab/pandas,rs2/pandas,rs2/pandas,datapythonista/pandas,GuessWhoSamFoo/pandas,pratapvardhan/pandas,nmartensen/pandas,TomAugspurger/pandas,gfyoung/pandas,GuessWhoSamFoo/pandas,jmmease/pandas,jreback/pandas,harisbal/pandas,zfrenchee/pandas,TomAugspurger/pandas,nmartensen/pandas,jreback/pandas,kdebrab/pandas,linebp/pandas,MJuddBooth/pandas,Winand/pandas,dsm054/pandas,winklerand/pandas,cbertinato/pandas,GuessWhoSamFoo/pandas,MJuddBooth/pandas,pratapvardhan/pandas,winklerand/pandas,kdebrab/pandas,zfrenchee/pandas,MJuddBooth/pandas,jreback/pandas,nmartensen/pandas,cbertinato/pandas,jmmease/pandas,linebp/pandas,gfyoung/pandas,louispotok/pandas,rs2/pandas,DGrady/pandas,cython-testbed/pandas,pandas-dev/pandas,pandas-dev/pandas,toobaz/pandas,pratapvardhan/pandas,Winand/pandas,Winand/pandas,cython-testbed/pandas,cython-testbed/pandas,dsm054/pandas,louispotok/pandas,DGrady/pandas,winklerand/pandas,cbertinato/pandas,jreback/pandas,jmmease/pandas,amolkahat/pandas,toobaz/pandas,toobaz/pandas,cbertinato/pandas,pratapvardhan/pandas,jorisvandenbossche/pandas,dsm054/pandas,amolkahat/pandas,cbertinato/pandas,DGrady/pandas,jreback/pandas,nmartensen/pandas,datapythonista/pandas,cython-testbed/pandas,nmartensen/pandas,harisbal/pandas,gfyoung/pandas,harisbal/pandas,nmartensen/pandas,gfyoung/pandas,pandas-dev/pandas,jorisvandenbossche/pandas,louispotok/pandas,pandas-dev/pandas,linebp/pandas,TomAugspurger/pandas,datapythonista/pandas,jmmease/pandas,winklerand/pandas,gfyoung/pandas,harisbal/pandas,MJuddBooth/pandas,jmmease/pandas,GuessWhoSamFoo/pandas,linebp/pandas,GuessWhoSamFoo/pandas,kdebrab/pandas,rs2/pandas,MJuddBooth/pandas,DGrady/pandas,dsm054/pandas,louispotok/pandas,zfrenchee/pandas,amolkahat/pandas,linebp/pandas,amolkahat/pandas,amolkahat/pandas,Winand/pandas,louispotok/pandas,zfrenchee/pandas,linebp/pandas,winklerand/pandas,TomAugspurger/pandas,Winand/pandas,zfrenchee/pandas,cython-testbed/pandas,Winand/pandas,dsm054/pandas,datapythonista/pandas