DOC: Add note to DataFrame.compare about only able to compare identically-labeled DataFrame objects
#### Location of the documentation
`pandas.core.frame::compare`

#### Documentation problem
Right now there is not a note/warning that you can only compare dataframes with the same shape and index labels.

#### Suggested fix for documentation
Add note: 

> Note: the method can only compare identically-labeled DataFrame objects, this means DataFrames with identical row and column labels.
