Feature/issue 668 next manual

#### Summary:

Add general manual updates collected from mailing lists since 2.3.

#### Intended Effect:

Bring manual up to date with user comments and version number.  

#### How to Verify:

You could compare the user comments to the changes in the manual.  

Also update version to 2.4.0;  Stan version update will update the build target.

I tested that all the new model code examples would compile in the context of larger models.

#### Side Effects:

None.

#### Documentation:

It's all doc. 

#### Reviewer Suggestions:

Anyone.