Reworking the dynamic table size section

For #594.

Here, I moved the text that covers the calculation of the dynamic header table size up into a new section.

Then, I reworded the text to reduce the direct dependency on HTTP/2 and its settings.  The emphasis is now on the using protocol setting a maximum, of which the HTTP/2 setting is just an example.

Apologies for the diff, I'll try to split the move commit out to make it more readable.