text changes to clear things up for newbies

I have gone through the text aimed at sources who might be submitting through a SecureDrop instance for the first time and tried to make the instructions a bit clearer. The major themes throughout these changes:
    
* Assume the reader is unfamiliar with the technologies used
* Emphasize the importance of a source remembering their code name and keeping it secret
* Make sure sources are aware that they can send documents, messages, or both through SecureDrop
* Make the step-by-step in `docs/user_manual.md` easier to follow
* Generally clean up syntax and fix some typos, etc.

Happy to field any feedback on these changes, thanks!