Tweaks from a doc readthrough

Mostly minor stuff from a readthrough.  Of particular note:

 - Regarding headers, "name-value pair" implies only one value; either it's a pair, and collections may have multiple instances with the same name; or it's a name with a collection of associated values.  Either is reasonable, but both is confusing.
 - Adjusting phrasing in SETTINGS_HEADER_TABLE_SIZE to better reflect that HPACK now lets the encoder change the table size at will.
 - Removing suggestion that a GOAWAY frame can get "lost", which can't happen in TCP; actual issue is that recipient may have acted on the previous GOAWAY and retried the requests the server says it didn't process.
Not sure about this one.  It doesn't quite work though, because the client isn't forced to open a new connection, it just means that the client will need a new connection for any new requests.  I think that distinction is important enough to make.  I'm leaning toward not including anything in this regard, honestly.