client: update obsolete comments

1) there is nothing like main thread since
   3c7e8bfbb4745315b7bcbf69fa746c3d6718c305 anymore, so remove
   it from documentation and update the doc accordingly.

2) use calling 'default queue' instead of 'main queue'. In the code
   we use display->default_queue, so it'll be easier the understand.

3) update some obsolete or unprecise pieces of documentation

v2. Not only remove out-of-date comment, but fix/remove more
    things across the wayland-client.[ch]

v3. fixes (rephrasing unclear paragraphs etc.)
    according to Pakka Paalanen notes (thanks)

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>
client: update documentation about threading

Remove out-dated documentation and add few more words
about this topic.

v2. replace a paragraph by better explanation from Pekka Paalanen
    fix other notes from reviewing

v3. fix typo

v4. fix flags for poll in an example

    add wl_display_cancel_read() to another example
    (so that user sees that it should be used)

    move proper use of wl_display_prepare_read
    before the explanation why it is wrong to use
    wl_display_displach

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>