Documentation for libunit

[alejandro-colomar]

Let's open this as the main thread for discussing the inclusion of documentation for libunit (a.k.a., Unit's C API).

I'll CC some interested parties:

Cc: @andreivasiliu
Cc: @junsoo999
Cc: @ac000
Cc: @artemkonev

And mention some related issues and PRs:

Link: #881
Link: #882
Link: #738
Link: #889

I've been writing documentation for a large part of libunit's API in the form of manual pages. It's still a draft, and probably has several typos, and other mistakes, but I hope it's useful to you, and at the same time you can help me by reading it and commenting anything that is not clear to you, or any mistakes you find.

It's written in the form of manual pages, which I compiled into a PDF book:

[Edit: I moved the PDF to the corresponding PR, so it can be reviewed together with the actual manual pages]

I expect that this is the form that will be more comfortable for review to the general audience, but if you prefer a set of manual page files (man(7) source), you can have a look at them in a git repository in my own server, as a fork of the Linux man-pages repo, in the unit branch: http://www.alejandro-colomar.es/src/alx/linux/man-pages/man-pages.git/?h=unit. The README and INSTALL files should tell you how to install them into your system so that you can run e.g. man nxt_unit_init. Note that some details such as link pages are not ready.

Please review the attached PDF (see link above).