lavf/http: fix/extend option descriptions

In particular, favor predicative form over nominal description of the set
parameter. This is more globally consistent.
doc/protocols: document http protocol options
doc/ffserver: rework introducing paragraphs of the "description" chapter

In particular, add some markup, fix empty line, and merge it with "How
does it work?" section.
doc/ffserver: remove paragraph in the introductory blurb

The paragraph is about what ffserver is not and where to look for other
information, but is pretty redundant and distracting, especially
considering the new organization of the documentation.
doc/ffserver: remove painfully outdated "What do I need?" section
doc/protocols: improve wording of a sentence in http docs

Suggested-By: Alexander Strasser