Flesh out some of the FIXMEs

[BjarniRunar]

(This includes @dkg 's current outstanding changes; if those get merged this will look much more reasonable. I'm very sorry if this is a crappy way to do this.)

Fill out some of the "FIXME" sections of the spec:

• Heading Capitalization-Tweaks
• Removed FIXME from Obscured Headers
• Removed FIXME regarding Enigmail protected-headers=v1 parameter
• Move "Message Rendering: Omitting..." into Message Interpretation
• New content:
  • Message Interpretation
  • Reverse-copying
  • Signature Invalidation
  • Reply/Forward Losing Subjects
  • Usability Impact of Reduced Metadata
  • Mailing List Header Modifications

[dkg]

Sweet, thanks for all this, @BjarniRunar . I've squashed these commits down to a single one (56697d8aeafb2eeb602686054fb573b38119be3f), so that "git blame" will not identify you as the source for some of the text that you added and removed in the back-and-forth discussion. it's all merged now though :)