Update exception-safe doc

Replace scope classes with structs.
Improve formatting.
Merge pull request #693 from ntrel/scope-class

Update exception-safe doc
Replace scope class with struct
Replace `length` in slices by `$`
Merge pull request #696 from ntrel/length-dollar

Replace `length` in slices by `$`
Merge pull request #694 from ntrel/scope-guard

Replace scope class with struct
Added instructions for building a single ddoc file without needing to clone other repositories
Merge pull request #698 from JinShil/patch-2

Added instructions for building a single ddoc file without needing to clone other repositories