C.62 etc: Mention copy-and-swap. Eliminate misinformation about self-move-assignment

[Quuxplusone]

Also mention the strong exception guarantee and maintainability; the "obviously correct" code in the old snippet was in fact NOT correct.

Also fix a ton of grammar issues in these sections.

[MikeGitb]

Didn't the standard have wording at some point that effectively forbid self move, because the implementation is allowed to assume that, if it has an r-value reference, that reference is the only one referencing that particular object? Has that been cleared up?