doc: Reword some of the client tutorial.
doc: Reword some of the client tutorial.

The overall meaning of the tutorial is the same. I added text describing frame types where it helps explain the callbacks better, e.g. introducing the HEADERS frame before on_header_callback().

Also fixed a couple of typos:
- Fixed ``reacb`` typo to ``readcb``
- Added parens to some functions, e.g. ``on_frame_recv_callback`` => ``on_frame_recv_callback()``

yuki-kodama/nghttp2,shines77/nghttp2,thinred/nghttp2,ohyeah521/nghttp2,yuki-kodama/nghttp2,minhoryang/nghttp2,icing/nghttp2,ohyeah521/nghttp2,serioussam/nghttp2,icing/nghttp2,lukw00/nghttp2,mixianghang/nghttp2,bxshi/nghttp2,minhoryang/nghttp2,minhoryang/nghttp2,wzyboy/nghttp2,dxq-git/nghttp2,wzyboy/nghttp2,lukw00/nghttp2,thinred/nghttp2,serioussam/nghttp2,dxq-git/nghttp2,ohyeah521/nghttp2,wzyboy/nghttp2,lukw00/nghttp2,yuki-kodama/nghttp2,mixianghang/nghttp2,wzyboy/nghttp2,minhoryang/nghttp2,thinred/nghttp2,lukw00/nghttp2,bxshi/nghttp2,icing/nghttp2,ohyeah521/nghttp2,serioussam/nghttp2,shines77/nghttp2,icing/nghttp2,lukw00/nghttp2,dxq-git/nghttp2,wzyboy/nghttp2,mixianghang/nghttp2,dxq-git/nghttp2,thinred/nghttp2,yuki-kodama/nghttp2,dxq-git/nghttp2,serioussam/nghttp2,serioussam/nghttp2,icing/nghttp2,mixianghang/nghttp2,yuki-kodama/nghttp2,mixianghang/nghttp2,minhoryang/nghttp2,minhoryang/nghttp2,shines77/nghttp2,lukw00/nghttp2,ohyeah521/nghttp2,dxq-git/nghttp2,serioussam/nghttp2,bxshi/nghttp2,bxshi/nghttp2,lukw00/nghttp2,thinred/nghttp2,wzyboy/nghttp2,thinred/nghttp2,shines77/nghttp2,bxshi/nghttp2,yuki-kodama/nghttp2,shines77/nghttp2,thinred/nghttp2,mixianghang/nghttp2,serioussam/nghttp2,bxshi/nghttp2,shines77/nghttp2,wzyboy/nghttp2,icing/nghttp2,ohyeah521/nghttp2,icing/nghttp2