Make GUI dialogs from s:prompt easier to read

- Add blank lines between the actual message and the note about how to disable the dialog. This makes it easier to read the important part of the dialog and ignore the note, if you have already decided that you don’t want to add that line to your vimrc.

  It makes an especially big improvement in MacVim, which follows Mac OS X’s convention displaying the first paragraph of a dialog in larger, bold text.

- Replace generic Yes and No button labels with ones specific to the dialog. It is a standard best practice to use buttons that describe the actual action when possible, instead of generic Yes/No buttons – see [User Experience Stack Exchange “Should I use Yes/No or Ok/Cancel on my message box?”](http://ux.stackexchange.com/q/9946/16961) for details.

  Thankfully, `s:prompt` was already written such that the labels can be changed without having to change any other code.

Before:

![a “restore your default editing session” dialog before my changes](https://cloud.githubusercontent.com/assets/79168/4622904/c53d20ac-5342-11e4-8442-8d8bf84eeb93.png)

After:

![a “restore your default editing session” dialog after my changes](https://cloud.githubusercontent.com/assets/79168/4622906/cb2ab8bc-5342-11e4-9c80-98fcf4010d10.png)