Apart from adding a section to describe the user scheme, this changeset
also does some much needed cleanup:

- fixed inverted reST targets
- fixed some paths
- avoided duplicating the same options listing five or six times
- added missing entries for C headers locations
- added documentation for --install-lib
- fixed a few misuses of the option role (see #9312), but not all (not
  worth the time, but will do it in packaging docs)
- fixed some markup

The paths fixes were done with an eye on the source code in the install
command, so they really describe what’s actually done.  The situation on
Mac OS X is rather messy: the fix for #8084 touched site and sysconfig,
but distutils does not use these files anymore since the Great Revert.
I suspect we have a mismatched stdlib at the moment, and the fix is not
even clear (see discussion on #8084).

site.USER_BASE and site.USER_SITE are now fully documented.  PEP 370 is
outdated with respects to the Mac framework situation, but the code in
sysconfig and the example in the 3.2 What’s New document helped me find
the right values to document for Mac OS X.

The command-line interface of the site module is also described in the
module docs.

The purpose of the usercustomize module is explained in the site docs,
with a gentle introduction in the tutorial (right after the section that
talks about PYTHONSTARTUP; a comment mentions it should be moved from
the tutorial to another file, but that will be another bug).

Various markup and wording improvements were made along the way in the
site module docs.  Duplicate and incomplete declarations of environment
variables have also been removed (the original bug report was actually
about these entries :).  The site module docs are still a bit messy;
I’ll see about improving them for #11553.

All these sections are copiously interlinked and findable from the doc
indexes.

Backporting two lines from the 3.x tests was enough to trigger the bug.
I also took the opportunity of making the logging call lazy.

Existing links have been updated to use the new reST role.  In some
files, I have also made cosmetic changes to the header.

This update includes new entries that apply to 2.7 too, mention of class
decorators, mention of nonlocal, notes about bytecode, markup fixes and
some rewrappings.  Future backports of changes should be slightly
easier.

No change was necessary to make the file build, contrary to cporting.rst
that uses markup like :c:func: that needs to be changed to :cfunc: in
2.7.  Future backports should be straightforward.

Georg added this role in our 3.2 doc tools and gave the greenlight for a
backport on python-dev.

This code is a simplified version of the 3.2 code; the version of Sphinx
used with Python 2.7 doesn’t have the function used to parse markup like
:role:`text to be displayed <text to be processed>` (I was persuaded it
was a standard reST construct, but it is actually a Sphinx innovation
that has to be supported explicitly in role code —I’ll be damned).  It
is thus not possible to write for example :source:`the NEWS file
<Misc/NEWS>`, but :source:`Misc/NEWS` will work.

resources (FD/zombie) when killed at the wrong time.

to 8-bit strings.

#12266: Fix str.capitalize() to correctly uppercase/lowercase titlecased and cased non-letter characters.

Confirm that the prime example is actually correct.  We get so many complaints about a "buggy example" on docs@python, let us hope this cuts them in half at least.