Use "round-trip" consistently

Just for the sake of consistency. It seems that both hyphenated and
non-hyphenated forms are correct (as listed in dictionaries), but prevailing
usage is hyphenated.
Drop "minimal" adjective from PUSH_PROMISE headers set

We don't define what "minimal" set of headers is and pretty much any headers
apply here.
Mark frame format figures as CDATA

Mostly a cosmetic change, that causes spacing preceeding the figures be
constant in some renderings, such as HTML.
Clarify that CONTINUATION can not be set after END_HEADERS

It was not clear that multiple CONTINUATION frames with END_HEADERS flag set
are invalid for a single headers block.
There's no "half closed" state

In 5.1, there's no "half closed" state defined. Throughout the document
"half closed" is used to describe "half closed (remote)" and "half closed
(local)" states. This change adjust remaining uses to the plural form.
Clarify what happens when a DATA packet is received in closed or idle connection

It was not able to find anything in the specification that would suggest
it's a protocol error.
Merge pull request #295 from lkundrak/lr-corrections

A couple of mostly minor corrections and clarifications