Fix #1219 - Mention FindProjectRoot behavior for Python in the documentation

autoload/ale/python.vim

@@ -15,6 +15,7 @@ let g:ale_virtualenv_dir_names = get(g:, 'ale_virtualenv_dir_names', [
 
 function! ale#python#FindProjectRootIni(buffer) abort
     for l:path in ale#path#Upwards(expand('#' . a:buffer . ':p:h'))
+        " If you change this, update ale-python-root documentation.
         if filereadable(l:path . '/MANIFEST.in')
         \|| filereadable(l:path . '/setup.cfg')
         \|| filereadable(l:path . '/pytest.ini')

doc/ale-python.txt

@@ -2,6 +2,31 @@
 ALE Python Integration                                     *ale-python-options*
 
 
+===============================================================================
+ALE Python Project Root Behavior                              *ale-python-root*
+
+For some linters, ALE will search for a Python project root by looking at the
+files in directories on or above where a file being checked is. ALE applies
+the following methods, in order:
+
+1. Find the first directory containing a common Python configuration file.
+2. If no configuration file can be found, use the first directory which does
+   not contain a readable file named `__init__.py`.
+
+ALE will look for configuration files with the following filenames. >
+
+  MANIFEST.in
+  setup.cfg
+  pytest.ini
+  tox.ini
+  mypy.ini
+  pycodestyle.cfg
+  flake8.cfg
+<
+
+The first directory containing any of the files named above will be used.
+
+
 ===============================================================================
 autopep8                                                  *ale-python-autopep8*
 
@@ -134,6 +159,8 @@ The minimum supported version of mypy that ALE supports is v0.4.4. This is
 the first version containing the `--shadow-file` option ALE needs to be able
 to check for errors while you type.
 
+`mypy` will be run from a detected project root, per |ale-python-root|.
+
 
 g:ale_python_mypy_executable                     *g:ale_python_mypy_executable*
                                                  *b:ale_python_mypy_executable*
@@ -292,6 +319,9 @@ g:ale_python_pylint_use_global                 *g:ale_python_pylint_use_global*
 ===============================================================================
 pyls                                                          *ale-python-pyls*
 
+`pyls` will be run from a detected project root, per |ale-python-root|.
+
+
 g:ale_python_pyls_executable                     *g:ale_python_pyls_executable*
                                                  *b:ale_python_pyls_executable*
   Type: |String|