[Bug Fix (and docs too)] : Do not select @develop version by default (#1933)

* This fixes a bug in concretization.  Before the recent change to the
algorithm, the intent was that the @develop version, although
"greater" than numberic versions, is never preferred BY DEFAULT over
numeric versions.

To test this... suppose you have a package with no `preferred=True` in
it, and nothing in `packages.yaml`, but with a `develop` version.  For
the sake of this example, I've hacked my `python/package.py` to work
this way.

Without bugfix (WRONG: user should never get develop by default):

```
  python@develop%clang@7.3.0-apple~tk~ucs4 arch=darwin-elcapitan-x86_64
      ...
```

With bugfix (RIGHT: largest numeric version selected):

```
  python@3.5.2%clang@7.3.0-apple~tk~ucs4 arch=darwin-elcapitan-x86_64
      ...
```

* Documented version selection in concretization algo.

* Fix typos

* flake8

0 files changed, 0 insertions, 0 deletions