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.
That's a good move, but I think that we need ...do not count toward the limit.