[FeauterRequest] doc: improving build-unix

While I tried to build Unison from scratch with the current
documentation I found some problems.

I would suggest some improvements of the relevant docu-part.
<http://www.cis.upenn.edu/~bcpierce/unison/download/releases/stable/unison-manual.html#build-unix>

1. make targets
 There is no explanation of the possible make targets. There is just a
 simple sentence with make-calls that should be made but without
 explanation what they are exactly doing. And how would "make install"
 affect a parallel existing Unison stable-version (or different
 version)?
 Or why are the doc-files (pdf & html) not "build" while "make"? There
 is a Makefile in the doc-folder but never touched as I saw.

2. foreign build
 There are to much informations about non-unison build-processes. How
 to build OCaml, some libs, etc. This confuses some readers (me! ;) )
 and creates redundant informations. A simple link to the information
 how to build something should be enough there.

3. build libs needful?
 The docu suggest that everything (ocaml, libs, ...) should be build
 from scratch. This is to much work and not needed (as I see in my
 case). I installed everything from my Ubuntu-repositories. The
 formulation of the text should be changed in a way that the reader
 know that he can but don't need to build all the needed parts.

4. detailed parts description
 Including my points below generally there should be more structured(!)
 information about all needed parts. A point-by-point list.

5. Pango
 There is no word about Pango. What is it why do I need it, etc. The
 gui-build doesn't work without it. And I did a lot of work to found
 out which of my repository packages I needed.

What do you thing about this modifications?
I would implement them.

Do I understand it right that all relevant doc-files are build from
"docs/unison-manual.tex"? So I should modify it. Or did I miss any
other files?