[PATCH] DOC: Fix typos in different subsections of the documentation

[Joseph Herlant]

Hi guys,

Fix typos found in the design-thoughts, internals and lua-api
subsections of the documentation.

Note: I split the whole documentation typos patch in 2 to make it more
readable but those can be merged if preferred of course.

The patch is attached, but if you want to view the diff online, you
can check: https://github.com/haproxy/haproxy/compare/master...aerostitch:doc_typos2

Thanks for your help,
Joseph