Flake8 3.0 was stopping once it found the current directory but the
historical behaviour (that we didn't intend to break) searched past
that (towards root) until it found one of the project/local config
file names that could be read.

Closes #181

Update documentation to separate parameter types

This is the documentation update separated from !80 which should be pretty uncontroversial. I already applied the comment on the documentation here.

See merge request !83

In Flake8 2.x we allowed people to use # noqa at the end of a multiline
string to ignore errors inside the string (e.g., E501). Being blissfully
ignorant of this, I never accounted for it in Flake8 3. This fixes the
oversight and allows multiline statements to have the # noqa at the end.

Closes #177

It updates the documentation to separate which parameters are static and
which are changed on each line. Using the latter parameters on plugins which
are only run once per file isn't very sensible.

The documentation for the `FileProcessor` class used `indect_char` while the
class itself uses the more sensible name `indent_char`. This updates both the
docstring as well as the documentation.

Yesterday we released the flake8-polyfill package to help with Flake8
compatibility issues. This adds documentation to Flake8 to help people
use that and to guide them towards it.